551 


,ESE   LIBRARY 


i'~ 

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IVERSITY    OF    CALIFORNIA 


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Received 
~,  Accessions  No. 


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Society.  By  WALTER  BAGEHOT.  Eighth  Edition. 

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VIII.  RESPONSIBILITY  in   MENTAL  DISEASE.     By  HEXRY 

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W.  STANLEY  JEVONS,  M.A.,  F.R.S.    Eighth  Edition. 

XVIII.  The  NATURE  of  LIGHT,  with  a  General  Account  of 
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XX.  FERMENTATION.     By  Professor  SCHUTZBNBERGER.     With 
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Membre  de  1'Institut.    Fourth  Edition. 

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BASTIAN,  M.D.     Third  Edition.    With  184  Illustrations. 

XXX.  The  ATOMIC  THEORY.  By  Professor  A.  WURTZ.  Trans- 
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XLI.  ANIMAL    INTELLIGENCE.     By  GEORGE  J.  KOMANES, 

LL.D.,  F.B.S.    Fourth  Edition. 

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XLIII.  DISEASES    of    MEMORY.      An   Essay   in   the   Positive 
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XLIV.  MAN  BEFORE   METALS.     By  N.  JOLY,   Correspondent 
de  1'Institut  de  France.    Fourth  Edition.    With  148  Illustrations. 

XLV.  THE    SCIENCE    of    POLITICS.        By    Prof.    SHELDON 
AMOS.    Third  Edition. 

XLVI.  ELEMENTARY     METEOROLOGY.       By   ROBEET   H. 

SCOTT.    Fourth  Edition. 

XLVII.  THE    ORGANS    of    SPEECH.       By    GEORG    HERMANN 

VON  MEYER.    With  47  Illustrations. 

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L.  JELLY  FISH,  STAB  FISH,  AND    SEA   URCHINS. 

Being  a  Research  on  Primitive  Nervous  Systems.    By  G-.  J.  ROMANES, 
LL.D.,  F.R.S. 

LI.  THE  COMMON  SENSE  OF  THE  EXACT 

SCIENCES.    By  the  late  WILLIAM  K.IXGDON  CLIFFORD.    Second  Edition. 
With  100  Figures. 

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By  FRANCIS  WARNER,  M.D.,  F.R.C.P.    With  50  Illustrations. 

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63  Illustrations. 

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PRIMEVAL  TIMES.    By  OSCAR  SCHMIDT.    With  51  Woodcuts. 

LV.  COMPARATIVE     LITERATURE.      By    H.     MACAULAY 

POSNETT,  LL.D. 

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By  Prof.  JOHN  MILNE.    With  38  Figures.    Second  Edition. 

LVII.  MICROBES,    FERMENTS,    and  MOULDS.     By  E.   L. 

TROUESSART.    With  107  Illustrations. 

LVIII.  GEOGRAPHICAL    and    GEOLOGICAL    DISTRIBU- 
TION of  ANIMALS.    By  Prof.  A.  HEILPRIN. 

LIX.  "WEATHER :  a  Popular  Exposition  of  the  Nature  of  Weather 
Changes  from  Day  to  Day.  By  the  Hon.  RALPH  ABERCROMBY.  With 
96  Figures.  Second  Edition. 

LX.  ANIMAL  MAGNETISM.     By  ALFRED  BLNET  and  CHARLES 

FERE. 

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tions of  all  the  Species  of  Fungi  hitherto  found  in  Britain  included  in  the 
Family,  and  Illustrations  of  the  Genera.  By  WILIJAM  PHILLIPS,  F.L.S. 

LXII.  INTERNATIONAL  LAW.  With  Materials  for  a  Code  of 
International  Law.  By  Professor  LEONE  LEVI. 

LXIII.  The  GEOLOGICAL  HISTORY  of  PLANTS.     By  Sir  J. 

WILLIAM  DAWSON.    With  80  Illustrations. 

LXIV.  THE  ORIGIN  OF  FLORAL  STRUCTURES  THROUGH 
INSECT  AND  OTHER  AGENCIES.  By  Prof.  G.  HENSLOW. 

LXV.  On  the  SENSES,  INSTINCTS,  and  INTELLIGENCE 
of  ANIMALS,  with  special  reference  to  INSECTS.  By 
Sir  JOHN  LUBBOCK,  Bart.,  M.P.  With  118  Illustrations. 


London:  KEG  AN  PAUL,  TRENCH,  &  CO.,  1  Paternoster  Square. 


THE 
INTERNATIONAL  SCIENTIFIC  SERIES. 


VOL.  LIX. 


WE  A  T  H  E  E 

A  POPULAR  EXPOSITION 

OF   THE 

NATURE  OF  WEATHER  CHANGES 
FROM  DAY  TO  DAY 


BY  THE 

HON.    RALPH    ABERCROMBY 

FELLOW  OF   THE   ROYAL   METEOROLOGICAL  SOCIETY,  LONDON; 

MEMBER  OF   THE   SCOTTISH   METEOROLOGICAL   SOCIETY;    AND    AUTHOR    OF 

"PRINCIPLES   OF  FORECASTING   BY    MEANS   OF  WEATHER  CHARTS" 


SECOND  EDITION 


LONDON 
KEG  AN  PAUL,  TRENCH  &  CO.,  1,  PATERNOSTER  SQUARE 

1888 


7 


(77«r  rights  of  translation  and  of  reproduction  are  reserved.) 


PREFACE. 


THE  object  of  this  work  is  the  same  as  that  of  other 
volumes  of  the  International  Scientific  Series,  to  which 
it  belongs— viz.  to  place  before  the  general  reader  a  short 
but  clear  picture  of  the  modern  /aspects  of  the  science  of 
which  it  treats.  \ 

With  this  view,  the  more  elementary  parts  of  the 
subject — weather  science — have  been  treated  of  in  the 
first  three  chapters  of  the  book,  while  the  more  difficult 
questions  are  reserved  for  the  later  portion  of  the  work. 
Though  this  method  of  treatment  involves  a  certain 
amount  of  repetition,  the  author  hopes  that  the  work 
may  thus  prove  acceptable  to  a  large  number  of  readers 
who  would  have  been  deterred  from  the  perusal  of  a  more 
formal  treatise. 

This  book  is  not  intended  to  be  in  any  way  an 
encyclopaedia  of  meteorology,  or  a  mere  repertory  of 
facts.  Our  endeavour  has  been  to  sketch  the  great 
principles  of  the  science  as  a  whole,  and  to  give  a  clear 

b 


VI  PREFACE 

picture  of  the  general  conclusions  as  to  the  actual  nature 
of  weather  to  which  meteorologists  have  been  led. 

Many  books  have  been  written  on  storms  and  climate, 
but  none  on  everyday  weather.  The  whole  of  this  work 
is  devoted  to  weather,  in  the  tropics  as  well  as  in  the 
temperate  zone. 

This  volume  is  not  a  mere  compilation  of  existing 
knowledge,  for  the  results  of  many  of  the  author's  original 
and  unpublished  researches  are  included  in  its  pages. 
Such,  for  instance,  as  the  explanation  of  many  popular 
prognostics ;  the  elucidation  of  the  general  principles  of 
reading  the  import  of  cloud-forms ;  the  classification  of 
those  cases  in  which  the  motion  of  the  barometer  fails  to 
foretell  correctly  the  coming  weather ;  and  the  character 
of  that  kind  of  rainfall  which  is  not  indicated  in  any  way 
by  isobaric  maps. 

Most  of  the  charts  are  derived  from  the  publications 
of  various  meteorological  offices ;  but  almost  all  the 
diagrams  have  been  drawn  for  this  work,  or  have  only 
appeared  in  some  of  the  author's  papers. 

Every  endeavour  has  been  made  to  do  justice  to  the 
discoverers  of  any  new  principle,  but  it  has  not  been 
considered  necessary  to  give  references  to  all  the  original 
authorities  in  a  popular  work. 

To  those  who  have  only  known  meteorology  as  a  dull 
branch  of  statistics,  the  perusal  of  these  pages  may 
perhaps  open  a  new  prospect  in  science,  and  a  new  vision 
to  the  mind. 


PREFACE.  Vll 

The  author  wishes  specially  to  acknowledge  the 
assistance  which  he  has  received  from  the  Meteorological 
Office  in  London,  and  the  United  States  Signal  Office,  by 
the  supply  of  the  material  contained  in  some  of  their 
various  publications  ;  and  also  the  courtesy  of  the  Council 
of  the  Koyal  Meteorological  Society  of  London,  in  lending 
him  the  blocks  which  have  illustrated  several  of  his  own 
papers. 

His  thanks  are  also  due  to  Mr.  E.  Ellery,  of  Melbourne 
Observatory,  and  Mr.  W.  E.  Cooke,  of  Adelaide  Observa- 
tory, for  information  and  diagrams  to  illustrate  Australian 
weather  and  forecasts ;  and  to  Mr.  H.  F.  Blanford, 
meteorological  reporter  for  the  Government  of  India,  for 
information  and  material  relating  to  the  nature  of  the 
monsoons. 


CONTENTS. 

PART  I.— ELEMENTARY. 
CHAPTER  I. 

INTRODUCTORY. 

PAGE 

Myths       ...             ...             ...             ...             ...  ...             ...         3 

Prognostics      ...             ...             ...             ...  ...             ...                 4 

The  barometer        ...             ...             ...             ...  ...             ...         4 

Statistics          ...             ...             ...             ...  ...             ...                 6 

Synoptic  charts      ...             ...             ...             ...  ...             ...         7 

Theoretical  developments              ...             ...  ...             ...               11 

Plan  of  the  book     ...             ...             ...             ...  ...             ...       12 

CHAPTER  II. 

WEATHER-PROGNOSTICS. 

Introduction            ...             ...             ...             ...  ...             ...       16 

Early  explanations         ...             ...             ...  ...             ...               17 

Modern  developments            ...             ...             ...  ...             ...       18 

Synoptic  charts               ...             ...             ...  ...             ...               18 

Relation  of  wind  and  weather  to  isobars            ...  ...             ...       23 

The  seven  fundamental  shapes  of  isobars  ...  ...             ...               25 

Cyclone-prognostics               ...             ...             ...  ...             ...       27 


Xll  CONTENTS. 

PAGE 

Variations  in  velocity  and  gradient  ...             ...             ...     187 

Relation  of  direction  to  gradient         ...  ...             ...             191 

Inclination  of  wind  to  isobars              ...  ...             ...             ...     192 

Calms                ...             ...             ...             ...  ...             ...             194 

Winds  in  southern  hemisphere             ...  ...             ...             ...     194 

General  remarks              ...              ...              ...  ...              ...              199 

Relation  of  force  to  velocity                 ...  ...             ...             ...     202 


CHAPTER  VII. 

HEAT   AND    COLD. 

Diurnal  isotherms ...              ...             ...  ...             ...             ...     204 

How  diurnal  modify  general  isotherms       ...  ...             ...             208 

Temperature-disturbance  of  a  cyclone  ...              ...             ...     213 

Sources  of  heat              ...             ...             ...  ...             ...             217 

Sources  of  cold        ...             ...             ...  ...             ...             ...     220 

The  "Blizzard"  and  the  "Barber"             ...  ...             ...             223 

Examples  of  daily  temperature-changes  over  Europe      ...  ...     226 

Forecasting  temperature               ...             ...  ...             *..             230 

Primary  and  secondary  effects  of  heat  ...             ...             ...     231 


CHAPTER   VIII. 

SQUALLS,   THUNDERSTORMS,    AND   NON-ISOBARIC    RAINS. 

Simple  squalls         ...             ...             ...             ...  ...             ...     234 

Thunder- squalls               ...             ...             ...  ...             ...             235 

Barometer  in  squalls  and  thunderstorms            ...  ...             ...     236 

Line-squalls     ...             ...              ...              ...  ...             ...             240 

Thunderstorms  associated  with  line-squalls      ...  ...             ...     245 

Thunderstorms  with  secondaries                  ...  ...             ...             254 

General  remarks      ...             ...             ...             ...  ...             ...     257 

Non-isobaric  rains          ...             ...              ...  ...             ...             259 

The  south-west  monsoon              ...             ...  ...             ...     259 


CONTENTS.  Xlll 


CHAPTER  IX. 

•PAMPEROS,    WHIRLWINDS,    AND   TORNADOES. 

PAGE 

Pamperoa...             ...             ...             ...             ...  ...             ...     263 

Whirlwinds      ...             ...             ...             ...             ...  ...             267 

Tornadoes                ...             ...             ...             ...  ...             ...     267 

Eelation  of  whirlwinds  to  cyclones             ...             ...  ...            277 

CHAPTER  X. 

LOCAL   VARIATION    OP   WEATHER. 

Nature  and  principles           ...             ...             ...  ...             ...     280 

Local  cloud     ...              ...             ...             ...             ...  ...             282 

„     rain                 ...             ...             ...             ...  ...             ...     284 

Mountain  rain                 ...             ...             ...              ...  ...             286 

Valley  rain               ...             ...             ...             ...  ...             ...     287 

Localization  of  hailstorms             ...             ...              ...  ...             288 

Tidal  showers          ...             ...             ...             ,..  ...             ...     291 

CHAPTER  XL 

DIURNAL   VARIATION    OP   WEATHER. 

Independence  of  diurnal  variations  and  general  changes  ...     294 

Diurnal  temperature     ...              ...             ...              ...  ...             294 

„       cloud           ...             ...             ...             ...  ...             ...     299 

„       rain     ...             ...             ...             ...             ...  ...             301 

„       wind           ...             ...             ...             ...  ...             ...     304 

„          „     velocity    ...             ...             ...             ...  ...             304 

„          „      direction          ...             ...             ...  ...             ...     306 

General  view  of  the  subject         ...             ...             ...  ...            310 

CHAPTER  XII. 

ANNUAL   AND    SECULAR   VARIATIONS. 

Seasonal  appearance  of  the  sky           ...             ...  ...             ...     312 

Recurrent  types  of  weather         ...              ...             ...  ...            312 


LIST   OF   ILLUSTRATIONS. 


FIG.  PAGB 

1.  Fundamental  shapes  of  isobars  ...  ...  „.             ...       25 

2.  Cyclone-prognostics              ...  ...  ...             ...               28 

3.  Weather  sequence  in  a  cyclone  ...  ...  ...             ...       40 

4.  "Weather  in  secondary  (synoptic)  ...  ...              ...               43 

5.  Weather  sequence  in  secondary  ...  ...  ...             ...       46 

6.  Anticyclone  prognostics       ...  ...  ...              ...               48 

7.  Wedge-shaped  isobar  prognostics  ...  ...             ...       54 

8.  Straight  isobar  prognostics ...  ...  ...             ...               60 

9.  Halo  prognostic — failure              ...  ...  ...             ...       66 

10.  „                 „              66 

11.  Cumulus  and  cirrus       ...             ...  ...  ...             ..*       73 

12.  Festooned  cumulus                ...  ...  ...             ...               78 

13.  Cumulus,  degraded  cumulus,  and  line  cumulus        ...  ...       80 

14.  Formation  of  cloud-stripes  ...  ...  ...             ...               85 

15.  Cloud-perspective          ...             ...  ...  ...             ...       87 

16.  Surface  and  highest  currents  over  cyclones  and  anticyclones        93 

17.  Converging  striated  cirrus-stripes  ...  ...              ...       97 

18.  Fleecy  cirro-cumulus            ...  ...  ...             ...             104 

19.  Strato- cumulus ;  roll  cumulus     ...  ...  ...             ...     110 

20.  Vertical  gradients  oVTSr  cyclone  and  anticyclone  ...  139 

21.  Cyclone  weather            ...             ...  ...  ...             ...     142 

22.  Anticyclone  weather             ...  ...  ...             ...             142 

23.  Weather  in  Y-depression              ...  ...  ...             ...     144 

24.  Wind  in  a  "col"   ...              ...  ...  ...             ...             148 

25.  A  meteogram                  ...             ...  ...  ...             ...     152 


PART  I. 
ELEMENTARY. 


WEATHER 


CHAPTEK  L 

INTRODUCTORY. 

THE  earliest  records  of  weather  among  every  nation  are 
to  be  found  in  those  myths,  or  popular  tales,  which,  while 
describing  rain,  cloud,  wind,  and  other  natural  phenomena 
in  highly  figurative  language,  refer  them  to  some  super- 
natural or  personal  agency  by  way  of  explanation. 

The  most  interesting  thing  about  these  mythical 
stories  is  the  remarkable  fidelity  with  which  they  reflect 
the  climate  of  the  country  that  gave  them  birth.  For 
example,  from  the  mythologies  of  Greece  and  Scandinavia 
we  can  almost  construct  an  account  of  the  climate  of 
those  two  countries  by  simply  translating  the  figurative 
phraseology  of  their  legends  into  the  language  of  modern 
meteorology. 

Many  survivals  of  mystic  speech  are  still  found  among 
popular  prognostics,  and  especially  in  cloud  names. 

In  England  and  Sweden  "  Noah's  Ark "  is  still  seen 
in  the  sky,  while  in  Germany  the  "  Sea-Ship  "  still  turns 


4  WEATHER. 

its  head  to  the  wind  before  rain.  In  Scotland  the  "  Wind- 
Dog  "  and  the  "  Boar's  Head  "  are  still  the  dread  of  the 
fisherman,  while  such  names  as  "  Goat's  Hair "  and 
"Mare's  Tails"  recall  some  of  the  shaggy  monsters  of 
antiquity. 

PROGNOSTICS. 

At  a  rather  later  period  of  intellectual  development, 
the  premonitory  signs  of  good  or  bad  weather  become 
formulated  into  short  sayings,  or  popular  prognostics.  A 
large  number  of  these  are  still  current  in  every  part  of 
the  world,  but  their  quality  and  value  is  very  varied. 
Some  represent  the  astrological  attitude  of  mind,  by 
referring  weather  changes  to  the  influence  of  the  stars  or 
phases  of  the  moon ;  others,  on  the  contrary,  are  very 
valuable,  and,  in  conjunction  with  other  aids  to  weather- 
forecasting,  prognostics  will  never  be  entirely  superseded, 
especially  for  use  on  board  ship.  Till  within  a  very 
recent  period,  their  science  and  explanation  had  hardly 
advanced  since  they  were  first  recorded.  In  many  cases 
the  prognostics  came  true";  when  they  failed,  no  explana- 
tion could  be  suggested  why  they  did  so ;  neither  could 
any  reason  be  given  why  the  same  weather  was  not  always 
preceded  by  the  same  signs.  A  halo  sometimes  precedes 
a  storm ;  why  does  it  not  always  do  so  ?  Why  is  rain 
sometimes  preceded  by  a  soft  sky,  and  sometimes  by  hard 
clouds  ? 

THE  BAROMETER. 

About  one  hundred  and  fifty  years  ago  the  barometer 
was  invented.  Very  soon  after  that  discovery,  observation 


INTRODUCTORY.  5 

showed  that,  in  a  general  way,  the  mercury  fell  before 
rain  and  wind,  and  rose  for  finer  weather.  Also  that  bad 
weather  was  more  common  when  the  whole  level  of  the 
barometer  was  low,  independent  of  its  motion  one  way  or 
the  other,  than  when  the  level  was  high.  But  as  with 
prognostics,  so  with  these  indications,  many  failures 
occurred.  Sometimes  rain  would  fall  with  a  high  or 
rising  barometer,  and  sometimes  there  would  be  a  fine 
day  with  a  very  low  or  falling  glass.  No  reason  could  be 
given  for  these  apparent  exceptions,  and  the  whole  science 
of  barometric  readings  seemed  to  be  shrouded  in  mystery. 

STATISTICS/ 

The  science  of  probabilities  came  into  existence  about 
the  commencement  of  this  century,  and  developed  the 
science  of  statistics.  By  this  method  the  average  readings 
of  meteorological  instruments,  such  as  the  height  of  the 
barometer  or  thermometer,  or  the  mean  direction  and 
force  of  the  wind,  at  any  number  of  places  were  calculated, 
and  the  results  were  sometimes  plotted  on  charts  so  as  to 
show  the  distribution  of  mean  pressure,  temperature,  etc., 
over  the  world. 

By  this  means  a  great  advance  was  made.  Besides 
giving  a  numerical  value  to  many  abstract  quantities,  the 
plotting  of  such  lines  as  the  isothermals  of  Dove  con- 
clusively showed  that  many  meteorological  elements 
hitherto  considered  capricious  were  really  controlled  by 
general  causes,  such  as  the  distribution  of  land  and  sea. 

Still  more  fruitful  were  these  charts  as  the  parents  of 
the  more  modern  methods  of  plotting  the  readings  of  the 


6  WEATHER. 

barometer  over  large  areas  at  a  given  moment,  instead  of 
the  mean  value  for  a  month  or  year.  We  shall  refer  to 
the  results  which  have  been  thus  obtained  more  fully 
presently.  Then  by  tabulating  statistics  of  the  relative 
frequency  of  different  winds  at  sea,  many  ocean  voyages 
— notably  those  across  the  "  doldrums,"  or  belt  of  calms 
near  the  equator— were  materially  shortened. 

Statistics  also  of  the  annual  amount  of  rainfall  became 
of  commercial  value  as  bearing  on  questions  of  the 
economic  supply  of  water  for  large  towns,  and  much 
valuable  information  was  acquired  as  to  the  dependence 
of  mortality  on  different  kinds  of  weather.  Of  more 
purely  scientific  interest  were  the  variations  of  pressure, 
temperature,  wind,  etc.,  depending  on  the  time  of  day,  or 
what  are  technically  known  as  diurnal  variations,  which 
were  brought  to  light  by  these  comparisons. 

This  branch  of  the  subject  is  known  as  "  Statistical 
Meteorology,"  and  has  advanced  very  little  since  it  was 
first  developed  by  Dove  and  Kaeintz. 

When  the  attempt  was  made  to  apply  statistics  to 
weather-changes  from  day  to  day,  it  was  found  that 
average  results  were  useless.  The  mean  temperature  for 
any  particular  day  of  the  year  might  be  50°,  if  deduced 
from  the  returns  of  a  great  many  years,  but  in  any  par- 
ticular year  it  might  be  as  low  as  40°,  or  as  high  as  60°. 
The  first  application  of  the  method  was  made  by  the 
great  Napoleon,  who  requested  Laplace  to  calculate  when 
the  cold  set  in  severely  over  Russia.  The  latter  found 
that  on  an  average  it  did  not  set  in  hard  till  January, 
The  emperor  made  his  plans  accordingly ;  a  sharp  spell 
of  cold  came  in  December,  and  the  army  was  lost. 


INTKODUCTOKY.  7 

It  has  now  been  thoroughly  recognized  that  statistics 
give  a  numerical  representation  of  climate,  but  little  or 
none  of  weather,  and  that  large  masses  of  figures  have 
been  accumulated,  to  which  it  is  difficult  to  attach  any 
physical  significance.  The  misuse  of  statistics  has  done 
much  to  bring  the  science  of  meteorology  into  disrepute. 

SYNOPTIC  CHAKTS. 

But  within  the  last  twenty  years  a  new  treatment  of 
weather  problems  has  been  introduced,  known  as  the 
synoptic  method,  by  which  the  whole  aspect  of  meteor- 
ology has  been  changed.  By  this  method,  a  chart  of  a 
large  area  of  the  earth's  surface  is  taken,  and  after  mark- 
ing on  the  map  the  height  of  the  barometer  at  each 
place,  lines  are  drawn  through  all  stations  at  which  the 
barometer  marks  a  particular  height.  Thus  a  line  would 
be  drawn  through  all  places  where  the  pressure  was 
30*0  inches,  another  through  all  where  it  was  29*8  inches, 
and  so  on  at  any  intervals  which  were  considered  neces- 
sary. These  lines  are  called  "  isobars,"  because  they  mark 
out  lines  of  equal  pressure.  When  these  charts  were 
first  introduced,  the  estimation  of  the  value  of  the  mean 
pressure  was  so  great  that,  instead  of  drawing  lines 
where  pressure  was  equal  at  the  moment,  they  were 
drawn  through  those  places  where  the  pressure  was 
equally  distant  from  the  mean  of  the  day  for  each  place. 
These  lines  were  called  "  is-abnormals ; "  that  is,  equal 
from  the  mean.  This  was,  however,  soon  abandoned,  for 
reasons  which  will  be  explained  farther  on  in  this  work. 
After  the  isobars  have  been  put  in,  lines  are  usually 


8  WEATHER. 

drawn  through  all  places  where  the  temperature  is  equal 
.at  the  moment.  These  are  called  "  isotherms,"  or  lines  of 
equal  temperature.  Then  arrows  to  mark  the  velocity 
and  direction  of  the  wind  are  inserted ;  and  finally 
.letters,  or  other  symbols,  to  denote  the  appearance  of  the 
sky,  the  amount  of  cloud,  or  the  occurrence  of  rain  or 
snow.  Such  a  chart  is  called  a  "synoptic  chart," 
because  it  enables  the  meteorologist  to  take  a  general 
view,  as  it  were,  over  a  large  area.  Sometimes  they 
are  called  "  synchronous  charts,"  because  they  are  com- 
piled from  observatioDS  taken  at  the  same  moment  of 
time. 

When   these   came   to   be   examined,   the   following 
important  generalizations  were  discovered : — 

1.  That  in  general  the  configuration  of  the   isobars 
assumed  one  of  seven  well-defined  forms. 

2.  That,  independent  of  the  shape  of  the  isobars,  the 
wind  always  took  a  definite  direction  relative  to  the  trend 
of  those  lines,  and  the  position  of  the  nearest  area  of 
low  pressure. 

3.  That  the  velocity  of  the  wind  was  always  nearly 
proportional  to  the  closeness  of  the  isobars. 

4.  That  the  weather — that  is  to  say,  the  kind  of  cloud, 
rain,  fog,  etc. — at  any  moment  was  related  to  the  shape, 
and  not  the  closeness,  of  the  isobars,  some  shapes  en- 
closing areas  of  fine,  others  of  bad,  weather. 

5.  That  the  regions  thus  mapped  out  by  isobars  were 
constantly   shifting   their   position,  so   that  changes   of 
weather  were  caused  by  the  drifting  past  of  these  areas 
of  good  or  bad  weather,  just  as  on  a  small  scale  rain  falls 
as  a  squall  drives  by.     The  motion  of  these  areas  was 


. 
INTRODUCTORY.  9 

found  to  follow  certain  laws,  so  that  forecasting  weather- 
changes  in  advance  became  possible. 

6.  That  sometimes  in  the  temperate  zone,  and  'habitu- 
ally in  the  tropics,  rain  fell  without  any  appreciable 
change  in  the  isobars,  though  the  wind  conformed  to 
the  general  law  of  these  lines. 

Observation  also  showed  that,  though  the  same  shapes 
of  isobars  appear  all  over  the  world,  the  details  of  weather 
within  them,  and  the  nature  of  their  motion,  are  modi- 
fied by  numerous  local,  diurnal,  and  'annual  variations. 
Hence  modern  weather  science  consists  in  working  out 
for  each  country  the  details  of  the  character  and  motion 
of  the  isobars  which  are  usually  found  over  it;  just  as 
the  geologist  finds  crumplings  and  denudation  all  over 
the  world,  and  works  out  the  history  of  the  physical 
appearance  of  his  own  scenery  by  studying  the  local 
development  of  these  agencies. 

So  far  the  science  rests  on  pure  observation — that  such 
and  such  wind  or  weather  comes  with  such  and  such  a 
shape  of  isobars.  But  it  has  been  found,  still  farther,  that 
the  seven  fundamental  shapes  of  isobars  are,  as  it  were, 
the  product  of  so  many  various  ways  in  which  an 
atmosphere  circulating  from  the  equator  to  the  poles 
may  move.  Just  as  the  motion  of  a  river  sometimes 
forms  descending  eddies  or  whirlpools,  sometimes  back- 
waters in  which  the  water  is  rising  upwards,  or  yet  at 
other  times  ripples  in  which  the  circulation  is  very  com- 
plex, so  it  now  appears  that  the  general  movement  of  the 
atmosphere  from  the  equator  to  the  pole  sometimes  breaks 
up  into  a  rotating  and  descending  movement  round  that 
configuration  of  isobars  known  as  an  anticyclone,  some- 


10  WEATHER. 

times  into  a  rotating  and  ascending  movement  round  that 
known  as  a  cyclone,  or  at  other  times  quite  in  a  different 
way  during  certain  kinds  of  squalls  and  thunderstorms. 

Isobars,  therefore,  represent  the  effect  on  our  barometers 
of  the  movements  of  the  air  above  us,  so  that  by  means  of 
isobars  we  trace  the  circulation  and  eddies  of  the  atmosphere. 

By  carrying  the  general  laws  of  physics  into  the 
conception  of  a  circulating  gas,  we  find  that  a  cold  mixed 
atmosphere  of  air  and  vapour  descending  into  a  warmer 
sqil  would  remain  clear  and  bright ;  while  a  similar 
atmosphere  rising  into  cooler  strata  would  condense  some 
of  its  vapour  into  rain  or  cloud.  It  is  by  reasoning  of 
this  nature  that  the  origin  of  some  of  the  most  beautiful 
and  complex  forms  of  clouds  has  been  discovered. 

Following  out  these  lines  of  research,  a  new  science 
of  meteorology  has  grown  up,  which  entirely  alters  the 
attitude  of  mind  with  which  we  regard  weather-changes, 
and  gives  rise  to  an  entirely  new  method  of  weather- 
forecasting  that  far  surpasses  all  previous  efforts,  and 
which  explains  and  develops  all  that  was  known  before. 

On  the  one  hand,  the  new  method  not  only  explains 
why  certain  prognostics  are  usually  signs  of  good  or  bad 
weather,  and  the  reason  why  the  indications  sometimes 
fail ;  but  also  the  reason  why  rain,  for  instance,  is  some- 
times foretold  by  one  prognostic,  and  sometimes  by  a 
totally  different  one. 

On  the  other  hand,  it  not  only  gives  a  more  extended 
meaning  to  all  the  statistics  which  partially  represent  the 
climate  of  a  place,  and  to  the  relation  of  the  diurnal  to 
the  general  changes  of  weather ;  but  it  also  enables  new- 
inferences  to  be  drawn,  which  had  hitherto  been  im- 


INTRODUCTORY.  11 

possible  from  some  observations,  and  explains  why  other 
sets  of  figures  must  always  remain  without  any  physical 
significance. 

THEORETICAL  DEVELOPMENTS. 

We  may  notice  here  an  attempt  which  has  been  made 
by  one  school  of  meteorologists  to  deduce  all  weather 
a  priori  from  changes  in  the  radiative  energy  of  the  sun ; 
that  is  to  say,  that  from  a  knowledge  of  greater  or  less  heat 
being  emitted  by  the  sun,  they  would  treat  the  consequent 
alteration  of  weather  as  a  direct  hydrodynamical  problem. 
Given  an  earth  surrounded  by  fifty  miles  of  damp  air,  and 
a  sun  at  varying  altitude,  and  of  varying  radiative  energy, 
deduce  from  that  all  the  diverse  changes  of  weather.  This 
is  doubtless  a  very  tempting  ideal,  for  there  is  no  doubt 
that  the  sun's  heat  is  the  prime  mover  of  all  atmospheric 
circulation ;  but  when  we  have  explained  what  the  nature 
of  weather-changes  is,  we  shall  see  that  there  is  little 
hope  that  this  method  will  ever  lead  to  satisfactory 
results. 

Other  meteorologists,  who  lay  less  stress  on  the  vary- 
ing power  of  the  sun,  have  taken  up  the  indications  of 
synoptic  charts,  and  endeavoured  to  construct  a  mathe- 
matical theory  of  cyclones  and  the  general  circulation  of 
the  atmosphere.  Ferrel,  Mohn,  Gulberg,  Sprung,  and 
others,  have  all  started  with  the  analysis  of  the  motion  of 
a  free  mass  of  air  on  the  earth's  surface,  first  given  by 
Professor  Ferrel,  and  worked  out,  from  that  and  other 
general  principles,  schemes  of  the  nature  and  propagation 
of  cyclones,  and  of  the  general  distribution  of  pressure 
over  the  world.  Though,  as  will  be  seen  hereafter,  the 


1  WEATHER. 

science  of  weather-forecasting  can  never  be  treated  mathe- 
matically, still  the  labours  of  these  writers  form  a  distinct 
branch  of  meteorology,  and  the  author  regrets  that  the 
scope  of  this  work  precludes  him  from  giving  a  chapter 
which  would  summarize  in  a  popular  manner  the  results 
that  they  have  obtained. 

This  is  the  deductive  portion  of  meteorology.  We 
shall  confine  this  work  entirely  to  the  inductive  branch 
of  the  science ;  and,  independent  of  any  theoretical  con- 
siderations, show  the  observed  association  of  different 
groups  of  phenomena,  and  the  generalizations  that  have 
been  arrived  at  by  observation  only. 

PLAN  OF  THE  BOOK. 

Though  a  vast  amount  of  work  has  been  given  to 
synoptic  meteorology  in  all  parts  of  the  world,  still  the 
results  obtained  by  different  investigators  remain  buried 
in  the  scattered  transactions  of  innumerable  societies,  and 
no  book  at  present  exists  which  contains  a  methodical 
statement  of  what  has  been  achieved.  Many  isolated 
principles  have  been  discovered,  but  no  attempt  has  been 
made  to  lay  down  the  broad  principles  of  the  science  of 
weather  as  a  whole.  The  object  of  this  work  is  to  supply 
that  want  by  putting  before  the  public  a  short  popular 
account  of  all  the  principal  results  which  have  been  dis- 
covered in  recent  years  by  means  of  synoptic  charts,  and 
of  their  bearing  not  only  in  modifying  all  our^  views  as  to 
the  nature  of  weather  at  all,  but  also  in  explaining  all 
that  was  previously  known.  We  shall  especially  en- 
deavour to  explain  the  general  principles  involved,  draw- 
ing our  illustrations  from  all  countries,  so  as  to  show 


INTRODUCTORY.  13 

what,  is  general  and  what  is  local,  and  to  give  a  truly 
International  character  to  this  work;  while  a  few  ex- 
amples of  British  weather  will  be  given  in  some  detail, 
so  as  to  demonstrate  how  the  minutest  weather-changes 
are  subordinate  to  general  laws. 

The  plan  of  this  book  will  be  as  follows.  We  shall 
commence  with  a  chapter  on  popular  weather-prognostics, 
so  as  to  introduce  some  of  the  simpler  portions  of  synoptic 
meteorology.  Clouds  and  cloud-prognostics  will  form  a 
chapter  by  themselves,  so  as  to  exhibit  the  great  develop- 
ment which  has  recently  been  made  in  the  interpretation 
of  their  indications.  So  far,  we  shall  confine  our  atten- 
tion to  weather  of  the  northern  temperate  zone  only. 

This  will  take  up  about  one-third  of  the  work,  and 
exhaust  the  more -popular  portions  of  the  subject.  We 
shall  then  have  to  plunge  more  deeply  into  the  details 
of  isobars,  and  explain  how  they  are  all  the  products  of 
different  forms  of  atmospheric  circulation.  From  them 
we  shall  pass  to  the  consideration  of  barograms  and 
meteograms  generally,  and  show  especially  how  the 
changes  in  the  shape  of  the  isobars,  as  seen  on  two  suc- 
cessive charts,  indicate  the  sequence  of  weather  as  ob- 
served in  any  one  place.  This,  which  is  the  fundamental 
point  of  all  synoptic  meteorology,  is  also  unfortunately 
the  most  difficult  to  grasp,  and  can  only  be  fully  realized 
after  considerable  practise.  Once,  however,  that  its  im- 
port is  fully  mastered,  the  remainder  of  the  work  will 
seem  comparatively  simple. 

We  shall  then  discuss  the  relation  of  both  the  velocity 
and  direction  of  the  wind  to  isobars,  and  after  that  the 
influence  of  different  shapes  of  isobars  in  modifying  the 


14  WEATHER. 

distribution  of  heat  and  cold  from  day  to  day  in  various 
parts  of  the  world. 

Squalls,  thunderstorms,  and  non-isobaric  rains  will  next 
engage  our  attention ;  and  a  short  chapter  on  Pamperos, 
whirlwinds,  and  Tornados  will  naturally  follow  next.  We 
shall  then  consider  the  local  influence  of  the  configuration 
of  the  earth's  surface  on  weather,  and  devote  a  whole  long 
chapter  to  the  diurnal  phenomena  of  weather,  with  special 
reference  to  the  manner  in  which  they  modify  the  weather 
that  characterizes  each  shape  of  isobars. 

From  this  we  shall  easily  be  led  to  comprehend  the 
nature  of  the  annual  fluctuations  of  weather,  and  of  those 
of  a  longer  period,  such  as  the  supposed  connection 
between  sunspots  and  rainfall. 

Having  thus  explained  what  we  may  call  the  com- 
ponents of  weather,  we  shall  be  ready  to  understand 
the  nature  of  sequences  or  spells  of  weather,  even  in 
the  most  variable  climates.  Our  illustrations  will  be 
drawn  chiefly  from  that  portion  of  the  northern  hemi- 
sphere which  lies  between  the  Urals  and  the  Eocky 
Mountains,  but  we  shall  also  include  some  examples  from 
the  monsoon  districts  of  India,  and  from  Australia,  so  as 
to  explain  the  nature  of  day  to  day  weather  in  the  tropics 
and  in  the  southern  hemisphere. 

When  this  is  done,  we  shall  have  completed  our 
description  of  the  nature  of  weather,  and  will  then  turn 
to  the  question  of  forecasting.  This  falls  readily  into 
two  distinct  problems:  1.  To  show  all  that  a  solitary 
observer,  with  a  barometer  and  his  eye-observations  on 
clouds  and  prognostics,  can  do  in  the  way  of  forecasting. 
In  this  chapter  we  shall  explain  fully  why  the  baro- 


INTRODUCTORY.  15 

meter  sometimes  appears  to  fail,  and  also  how  much  the 
older  knowledge  can  be  increased  by  a  knowledge  of 
synoptic  charts.  The  space  at  our  disposal  will  not,  how- 
ever, permit  us  to  explain  the  modern  developments  of 
the  principles  of  handling  ships  in  hurricanes,  which 
would  naturally  come  in  this  chapter. 

2.  To  show  what  a  meteorologist  can  do,  seated  in 
a  central  bureau,  with  telegraphic  communication  in  all 
directions,  and  who,  after  making  a  synoptic  chart,  and 
combining  it  with  every  other  modern  aid,  issues  tele- 
graphic forecasts  to  all  parts  of  the  country.  This  is  the 
highest  problem  of  meteorology. 


16  WEATHER. 


CHAPTER  II. 

WEATHER-PROGNOSTICS. 

INTRODUCTION. 

THE  second  stage  in  the  history  of  meteorology,  after  the 
mythic  phase  has  been  passed,  is  the  collection  of 
numerous  observations  on  the  appearance  of  the  sky,  the 
movements  of  animals,  etc.,  before  rain  or  fine  weather 
into  the  form  of  short  sayings,  which  are  usually  known 
as  popular  prognostics.  For  instance,  halos  round  the 
sun,  or  swallows  flying  low,  are  known  all  over  the  world 
very  frequently  to  precede  rain.  On  the  other  hand,  a 
copious  deposition  of  d'ew,  or  a  white  silvery  moon,  are 
equally  widely  known  as  precursors  of  fine  weather. 

One  of  the  earliest  collections  of  prognostics  is  found  in 
the  "  Diosemeia  "  of  Aratus,  a  Greek  who  flourished  in 
Macedonia  and  Asia  Minor  about  270  B.C.  The  principal 
interest  attached  to  his  work  is  that  many  of  his  prog- 
nostics were  incorporated  by  Virgil  in  his  Georgics,  and 
that  from  them — through  the  medium  of  the  Latin 
monks,  during  the  revival  of  learning  in  the  Middle  Ages 
— a  very  considerable  number  have  been  translated  into 


WEATHER-PROGNOSTICS.  17 

modern  European  languages,  and  are  in  current  use  at  the 
present  time. 

EARLY  EXPLANATIONS. 

From  classic  times,  down  to  the  commencement  of 
this  century,  it  can  hardly  be  said  that  this  branch  of 
meteorology  made  any  advance.  Few,  if  any,  new  prog- 
nostics had  been  discovered,  and  neither  their  physical 
explanation  nor  their  meteorological  significance  had 
been  found  out.  But  about  eighty  years  ago,  some 
physical  explanations  were  given.  It  was  found  that  the 
air  always  contained  a  certain  quantity  of  uncondensed 
vapour,  and  means  were  invented  for  measuring  this 
amount  accurately.  From  this,  the  nature  and  conditions 
of  the  formation  of  dew  were  discovered,  and  also  that 
before  many  cases  of  rain  the  air  became  more  charged 
with  vapour.  This  latter  fact  gave  the  explanation  of 
several  rain-prognostics.  For  instance,  when  walls  sweat, 
stones  grow  black,  and  clouds  form  on  hilltops,  rain  may 
be  expected  almost  all  the  world  over. 

But  even  when  these  reasons  had  been  discovered,  the 
science  flagged.  A  large  number  of  rain-prognostics 
could  not  be  shown  by  any  means  to  depend  on  an  increase 
of  moisture,  and,  as  vapour  cannot  grow  in  the  air,  some 
explanation  was  needed  to  account  for  its  variable  quan- 
tity. And  even  when,  in  a  general  way,  the  prognostic 
had  been  explained,  no  clue  whatever  had  been  found  for 
what  we  may  call  the  meteorological  significance.  What 
was  the  relation  of  the  damp  to  the  rain  ?  Why  did  the 
prognostic  sometimes  fail?  Why  are  there  many  rain- 
prognostics  associated  with  a  tolerably  dry  air  ?  Why  is 

c 


18  WEATHER. 

not  all  rain  preceded  by  the  same  set  of  prognostics? 
To  all  these  questions  no  answer  could  be  given.  Prog- 
nostics had  almost  fallen  into  disrepute  ;  they  were  con- 
sidered no  part  of  science,  and  had  been  supposed  to  be 
only  suitable  for  rustics  and  sailors. 

MODERN  DEVELOPMENTS. 

So  the  subject  remained  till  the  introduction  of 
synoptic  charts.  Then  it  was  soon  seen  that  in  Temperate 
regions  the  broad  features  of  weather  depend  on  the 
shape  of  the  isobaric  lines,  and  later  on  it  was  shown — 
the  author  believes,  mainly  by  himself — that  nearly  all 
prognostics  have  a  definite  place  in  some  shape  of  isobars, 
and  that  all  the  above  questions,  formerly  insoluble, 
receive  a  ready  explanation.  It  has  also  been  demon- 
strated that  prognostics  can  never  be  superseded  for  use 
on  board  ship,  and  that  even  in  the  highest  developments 
of  weather-forecasting  by  means  of  electric  telegraph, 
prognostics  often  afford  most  valuable  information.  But 
before  we  attempt  to  explain  how  this  is  done,  we  must 
introduce  the  reader  into  the  elements  of  synoptic  meteor- 
ology. 

SYNOPTIC  CHARTS. 

Synoptic  meteorology  is  that  part  of  the  science  which 
deals  with  the  results  obtained  by  constructing  synoptic 
charts.  Formerly,  all  meteorology  was  deduced  from  the 
changes  which  took  place  in  the  instrumental  readings 
at  any  one  place  during  any  interval  of  time,  say  one 
day.  For  instance,  a  great  deal  had  been  discovered  as 


WEATHER-PROGNOSTICS.  19 

to  the  connection  between  a  falling  or  rising  barometer 
and  the  accompanying  rain  or  wind.  Synoptic  charts,  on 
the  contrary,  are  constructed  by  taking  the  readings  of 
any  instrument  (say  the  barometer),  or  any  observations 
on  the  sky  or  the  weather  (say  where  rain  is  falling,  or 
cloud  or  blue  sky  is  seen),  at  a  large  number  of  places 
at  the  same  moment  (say  8  a.m.  at  Greenwich).  A  map 
of  the  area  or  district  from  which  the  observations  have 
been  received  is  then  taken,  the  barometer-readings  are 
marked  down  over  their  respective  places,  and  then  lines 
are  drawn  through  all  the  stations  where  the  pressure  is 
equal.  For  instance,  through  all  the  places  where  the 
pressure  is  29*9  inches  (760  mm.),  and  again  at  convenient 
intervals,  generally  of  about  two-tenths  of  an  inch,  say 
29-7  ins.  (755  mm.),  29'5  ins.  (750  mm.),  and  so  on.  These 
lines  are  called  isobaric  lines,  or  more  shortly  isobars — 
that  is,  lines  of  equal  atmospheric  weight  or  pressure. 
This  method  of  showing  the  distribution  of  pressure  by 
isobars  is  exactly  analogous  to  that  of  marking  out  hills 
and  valleys  by  means  of  contour  lines  of  equal  altitude. 

Similarly,  the  places  which  report  rain,  cloud,  blue  sky, 
etc.,  are  marked  with  convenient  symbols  to  denote  these 
phenomena.  In  Great  Britain,  a  system  known  as  Beau- 
fort's weather-notation  is  exclusively  used.  It  is  as  follows: 
This  will  be  useful,  as  it  is  employed  in  all  our  charts. 

BEAUFOKT'S  NOTATION  OF  WEATHER. 

SYMBOL. 

b  Blue  sky,  whether  with  clear  or  hazy  atmosphere. 

c  Clouds  (detached). 
d  Drizzling  rain. 
/  Fog. 
g  Gloomy,  very. 


20  WEATHER. 

SYMBOL. 
h  Hail. 
I  Lightning. 

m  Misty,  hazy  atmosphere. 

o  Overcast,  the  whole  sky  being  covered  with  au  impervious  cloud, 
p  Passing,  temporary  showers. 
q  Squally. 

r  Rain,  continued  rain, 
s  Snow. 
t  Thunder. 

u  Ugly,  threatening  appearance  of  the  weather. 
v  Visibility,  whether  the  sky  be  cloudy  or  not. 
w  Dew. 

We  should  remark  here  that,  though  in  common 
parlance  the  word  "  weather  "  is  used  collectively  for  the 
sum  of  every  meteorological  element,  wind,  rain,  heat, 
cold,  etc.,  in  this  work,  and  in  all  synoptic  charts, 
"weather"  is  used  in  a  more  restricted  sense  to  denote 
whether  the  actual  appearance  of  the  sky  is  blue,  cloudy 
or  otherwise,  and  whether  rain,  snow,  hail,  etc.,  are  falling. 

Then  arrows  are  placed  over  each  observing  station, 
with  a  number  of  barbs  and  feathers  which  roughly 
indicate  the  force  of  the  wind.  By  an  international 
convention,  the  arrows  always  fly  with  the  wind  ;  that  is  to 
say,  they  do  not  face  the  wind  like  the  pointer  of  a  wind- 
vane.  The  scale  of  force  usually  adopted  is  that  of  Beau- 
fort, which  is  given  opposite.  It  will  be  observed  that  this 
is  a  practical  scale,  based  on  the  amount  of  canvas  a  ship 
can  carry.  At  sea  this  is  certainly  a  better  gauge  than 
any  instrumental  readings,  though  there  is  always  a 
certain  disagreement  in  the  estimate  of  different  observers. 
For  land-observations,  and  those  unacquainted  with  ships, 
an  equivalent  of  miles  per  hour  and  metres  per  second 
is  given. 


WEATHER-PKOGNOSTICS. 


21 


BEAUFORT'S  SCALE  OF  WIND. 


FORCE. 

0.  Calm. 

1.  Light  air. 

2.  Light  breeze.       \ 

3.  Gentle  breeze. 

I 

4.  Moderate  breeze. > 

5.  Fresh  breeze.       > 

6.  Strong  breeze. 

7.  Moderate  gale. 

8.  Fresh  gale. 

9.  Strong  gale. 

10.  Whole  gale. 

11.  Storm. 

12.  Hurricane. 


Or  just  sufficient  to  give  steerage 
way  


Or  that  in  which  a 
well  -  conditioned 
man-of-war,  with 
all  sail  set,  and  clean 
full,  would  go  in 
smooth  water,  from 
n  ,,,  /Royals,  etc. 

towMfh    Wfc"" 


1  to  2  knots 


fn  4. 


5  to  6  knots 


topsails 


by. 


le-reefed    topsails, 
etc 

IClose  -  reefed    topsails 

and  courses 

Or    that    with    which    she    could 
scarcely  bear  close-reefed   main- 
topsail  and  reefed  foresail 
Or  that  which  would  reduce  her  to 

storm-stay  sails    ... 
Or  that  which  no  canvas  could  with- 
stand 


VELOCITY. 

Miles    Metres 
per        per 
hour,   second. 

3   1-34 

8   3-6 

13   5-82 

18   8-1 

23  10-3 
28  12-5 

34  15-2 

40  17-9 

48  21-5 

56  25-0 

65  29-0 
75  33-5 
90  40-0 


When  all  this  is  done,  we  can  see  at  a  glance  whether 
or  how  wind,  rain,  cloud,  and  blue  sky  are  connected  with 
the  shape  of  the  isobars.  In  fact,  a  synoptic  chart  gives  us, 
as  it  were,  a  bird's-eye  view  of  the  weather  at  the  particular 
moment  for  which  the  chart  is  constructed,  over  the  whole 
district  from  which  reports  have  been  received.  Suppose, 
now,  that  after  an  interval  of  twenty-four  hours  another 


22  WEATHER. 

chart  is  constructed  from  observations  taken  over  the 
same  area,  then  we  generally  find  that  the  shape  of  the 
isobars  and  the  position  of  the  areas  of  high  and  low 
pressure  have  considerably  changed,  and  with  them  the 
positions  of  those  areas  where  the  weather  is  good  or  bad. 
For  instance,  suppose  that  at  8  a.m.  on  one  morning  we 
find  pressure  low  over  Ireland  and  high  over  Denmark, 
with  rain  over  Ireland,  cloud  over  England,  and  blue  sky 
in  Denmark ;  and  that  by  8  a.m.  on  the  following  day  we 
find  that  the  low-pressure  area  has  advanced  to  Denmark, 
and  that  a  new  high  pressure  has  formed  over  Ireland, 
with  rain  in  Denmark,  broken  sky  in  England,  and  blue 
sky  in  Ireland;  suppose,  too,  that  the  record  of  the 
weather,  say  in  London,  for  those  twenty-four  hours  had 
been  as  follows : — cloudy  sky,  followed  by  rain,  after  which 
the  sky  broke ; — how  can  an  inspection  of  the  two  charts 
help  us  to  explain  the  weather  as  observed  in  London  during 
that  day  ?  'Our  bird's-eye  view  would  show  that  the  rain- 
area  which  lay  over  Ireland  in  the  morning  had  drifted 
during  the  day  over  England,  including  London,  and 
covered  Denmark  by  next  morning.  It  would  also  tell 
us  that  the  position*  of  the  rain  was  identified  with, 
and  moved  along  with  the  low  pressure.  This  is  the 
fundamental  idea  of  all  synoptic  meteorology,  but  one 
which  can  only  be  thoroughly  grasped  after  a  consider- 
able experience  in  tracing  actual  cases.  It  is  so  different 
looking  at  the  "ups"  and  "downs"  of  the  barometer 
when  they  are  marked  on  a  diagram,  and  then  at  any 
two  synoptic  charts  which  refer  to  the  same  period,  that 
it  is  very  difficult  at  first  to  see  any  connection  at  alL 
In  fact,  deductions  from  barograms — as  such  barometric 


WEATHER-PROGNOSTICS.  23 

traces  are  called — and  deductions  from  synoptic  charts 
are  so  apparently  unconnected  that  they  have  hitherto 
been  almost  treated  as  different  branches  of  meteorology. 
One  main  feature  of  this  book  will  be  our  endeavour  to 
collate  these  deductions  together,  and  to  show  how 
changes  in  the  charts  for  a  large  district  are  simultane- 
ously shown  by  fluctuation  in  the  instrumental  readings 
at  any  one  place.  It  must  be  borne  in  mind,  however, 
that  the  whole  aim  and  object  of  meteorology  is  to  ex- 
plain weather  as  it  occurs  at  any  place ;  that  is,  what 
successive  changes  each  individual  observer  will  ex- 
perience. Synoptic  charts  are  only  a  means  to  this  end. 

KELATIONS  OF  WIND  AND  WEATHER  TO  ISOBARS. 

Such,  then,  is  a  synoptic  chart.  Many  thousands 
have  been  constructed  for  all  parts  of  the  world,  and  by 
comparing  them  the  following  important  generalizations 
have  been  arrived  at : — 

1.  That  in  general  the  configuration  of  the  isobars 
takes  one  of  seven  well-defined  forms. 

2.  That,  independent  of  the  shape  of  the  isobars,  the 
wind  always  takes  a  definite  direction   relative   to  the 
trend  of  these  lines,  and  the  position  of  the  nearest  area 
if  low  pressure. 

3.  That  the  velocity  of  the  wind  is  always  nearly  pro- 
portional to  the  closeness  of  the  isobars. 

4.  That  the  weather — that  is  to  say,  the  kind  of  cloud, 
rain,  fog,  etc. — at  any  moment  depends  on  the  shape,  and 
not  the   closeness,   of  the    isobars,   some   shapes   being 
associated  with  good  and  others  with  bad  weather. 


WEATHER. 

5.  That  the  regions  thus  mapped  out  by  the  isobars 
were  constantly  shifting  their  position,  so  that  changes 
of  weather  were  caused  by  the  drifting  past  of  these  areas 
of  good  or  bad  weather,  just  as  on  a  small  scale  rain  falls 
as  a  squall  drives  by.     The  motion  of  these  areas  was 
found  to  follow  certain  laws,  so  that  forecasting  weather 
changes  in  advance  became  a  possibility. 

6.  That  in  the  temperate  zones  sometimes,  and  habitu- 
ally in   the   tropics,  rain   fell   without   any  appreciable 
change  in  the  isobars,  though  the  wind  conformed  more 
regularly  to  the  general  law  of  these  lines.     This  class  of 
rainfall  will  be  called  throughout  this  work  "  non-isobaric 
rain." 

It  will  be  convenient  to  take  first  the  broad  features 
of  the  relation  of  wind  to  isobars,  which  are  as  follows : — 

First  as  regards  direction.  The  wind  in  all  cases  is 
not  exactly  parallel  to  the  isobar,  but  inclined  towards 
the  nearest  low  pressure  at  an  angle  of  from  30°  to  40°.  If 
you  stand  with  your  back  to  the  wind,  the  lowest  pressure 
will  always  be  on  your  left  hand  in  the  northern  hemi- 
sphere, and  on  your  right  in  the  southern  hemisphere. 
This  is  what  is  commonly  known  as  "Buys  Ballot's 
Law." 

Then  as  to  velocity.  All  we  need  say  here  is  that 
the  velocity  is  roughly  proportional  to  the  closeness  of 
the  isobars,  and  that  the  measure  of  the  closeness  is 
called  the  barometric  gradient,  for  in  our  chapter  on 
wind  and  calm  we  will  give  all  necessary  details  on  this 
branch  of  the  subject. 

The  upshot  of  these  two  principles  is,  that  if  you  give 
a  meteorologist  a  chart  of  the  world  with  the  isobars  only 


WEATHER-PROGNOSTICS. 


25 


marked   on   it,  he   can   put   in   very  approximately  the 
direction  and  force  of  the  wind  all  over  the  globe. 

When  we  have  explained  the  relation  of  weather  to 
the  shapes  of  isobars,  we  shall  see  that  he  could  also  write 
down  very  nearly  the  kind  of  weather  which  would  be 
-experienced  everywhere. 

THE  SEVEN  FUNDAMENTAL  SHAPES  OF  ISOBARS. 

Then  as  to  the  shapes  themselves. 

In  Fig.  1  we  give  in  a  diagrammatic  form  the  broad 


Cyclone 


FIG.  1. —  The  seven  fundamental  shapes  of  isobars. 

features  only  of  the  distribution  of  pressure  over  the 
North  Atlantic,  Europe,  and  the  eastern  portions  of  the 
United  States  on  February  27,  1865.  Coast-lines  are 
omitted  so  as  not  to  confuse  the  eye,  so  also  are  lines  of 
latitude  and  longitude;  but  the  foot-note  at  the  bottom 
of  the  figure  represents  the  equator,  and  the  top  of  the 
diagram  would  be  on  the  Arctic  Circle.  All  pressures 


26 


WEATHER. 


of  and  under  29 '9  ins.  (760  mm.)  are  shown  with  dotted 
lines,  so  that  the  eye  sees  at  a  glance  the  broad  dis- 
tribution of  high  or  low  pressure.  The  whole  seven 
fundamental  shapes  of  isobars  will  be  found  there. 

Looking  at  the  top  of  the  diagram,  we  see  two  nearly 
circular  areas  of  low  pressure,  round  which  the  isobars  are 
rather  closely  packed.  Such  areas,  or  rather  the  con- 
figurations of  isobars  which  enclose  them,  are  called 
"cyclones,"  from  a  Greek  word  meaning  a  circle,  because 
they  are  nearly  circular,  and,  as  we  shall  see  presently,  the 
wind  blows  nearly  in  a  circle  round  their  centre. 

Just  south  of  one  of  the  cyclones,  the  isobar  of  29'9  ins. 
(760  mm.)  forms  a  small  sort  of  nearly  circular  loop, 
enclosing  lower  pressure ;  this  -is  called  a  "  secondary 
cyclone,"  because  it  is  usually  secondary  or  subsidiary 
to  the  primary  cyclones  above  described. 

Further  to  the  left  the  same  isobar  of  29'9  ins.  bends 
itself  into  the  shape  of  the  letter  V,  also  enclosing  low 
pressure;  this  is  called  a  "Y-shaped  depression,"  or, 
shortly,  a  «  V." 

Between  the  two  cyclones  the  isobar  of  29*9  ins.  pro- 
jects upwards,  like  a  wedge  or  an  inverted  letter  V.,  but 
this  time  encloses  high  pressure ;  this  shape  of  lines  i& 
called  a  "  wedge." 

Below  all  these  we  see  an  oblong  area  of  high  pressure, 
round  which  the  isobars  are  very  far  apart ;  this  is  called 
an  "  anticyclone,"  because  it  is  the  opposite  to  a  cyclone 
in  everything — wind,  weather,  pressure,  etc. 

Between  every  two  anticyclones  we  find  a  furrow, 
neck,  or  "  col  "  of  low  pressure  analogous  to  the  col  which 
forms  a  pass  between  two  adjacent  mountain-peaks. 


WEATHER-PKOGNOSTICS.  27 

Lastly,  as  marked  in  the  lower  edge  of  the  diagram, 
isobars  sometimes  run  straight,  so  that  they  do  not  include 
any  kind  of  area,  but  represent  a  barometric  slope 
analogous  to  the  sloping  sides  of  a  long  hill. 

We  may  forestall  succeeding  chapters  so  far  as  to  say 
that  the  cyclones,  secondaries,  V's,  and  wedges  are  usually 
moving  towards  the  east  at  the  rate  of  about  twenty  miles 
an  hour ;  but  that  the  anticyclones,  on  the  contrary,  are 
usually  stationary  for  days,  and  sometimes  for  months 
together. 

We  should  also  note  that,  though  the  general  principles 
of  prognostics  and  the  broad  features  of  the  weather  in 
each  of  these  shapes  of  isobars  are  the  same  all  over  the 
world,  the  minute  details  which  we  intend  to  give  now 
apply  to  Great  Britain  and  the  temperate  zones  only. 

We  will  now  take  the  five  more  important  shapes 
separately,  and  detail  the  kind  of  wind  and  weather  which 
is  experienced  in  different  parts  of  each  of  them.  From 
this  we  shall  be  led  to  the  explanation  of  the  nature  of 
popular  prognostics.  The  account  of  "  V's  "  and  "  cols  " 
will  be  reserved  for  our  chapter  on  isobars,  as  no  special 
prognostics  are  grouped  round  these  two  forms. 

CYCLONE- PROGNOSTICS. 

We  will  begin  with  these  as  they  are  by  far  the  most 
important.  In  Fig.  2  we  give  a  diagram  on  which  we 
have  written  in  words  the  kind  of  weather  which  would 
be  found  in  every  portion  of  a  typical  cyclone;  arrows 
also  show  the  direction  of  the  wind  relative  to  the  isobars 
and  to  the  centre. 


28  WEATHER 

First  let  us  look  at  the  isobars.  We  find  that  they 
are  oval,  and  that  they  are  not  quite  concentric,  but  the 
centre  of  the  inner  one  we  will  call  the  centre  of  the 
cyclone.  Now  observe  the  numbers  attached  to  the  isobars ; 
the  outer  one  is  3OO  ins.  (762  mm.),  the  inner  one  29'0 
ins.  (737  mm.).  But  suppose  the  outer  one  was  the  same, 


Blue 


Cirrus 
FIG.  2. — Cyclone  prognostics. 


but  the  inner  one  was  29*5  (755  mm.)  We  should  then 
have  two  cyclones,  differing  in  nothing  but  depth';  that  is, 
in  the  closeness  of  the  isobars,  or  the  steepness  of  the 
barometric  slope.  Observation  has  shown  that  under 
these  circumstances  the  general  character  of  the  weather 
and  the  direction  of  the  wind  everywhere  would  be  the 
same ;  the  only  difference  would  be  that  the  wind  would 


WEATHER-PROGNOSTICS.  29 

blow  a  hard  gale  in  the  first,  and  only  a  moderate  breeze 
in  the  second  case  ;  and  that  what  was  a  sharp  squall  in 
the  one  would  be  a  quiet  shower  in  the  other.  This  is 
one  of  the  fundamental  principles  of  synoptic  meteorology 
— that  the  character  of  the  weather  and  direction  of  the 
wind  depend  entirely  on  the  shape  of  the  isobars,  while 
the  force  of  the  wind  and  intensity  of  the  character  of 
the  weather  depend  only  on  the  closeness  of  the  isobars. 

The  difference  in  the  details  of  the  weather  in  a 
cyclone,  or  any  other  isobaric  shape  which  are  due  to 
difference  in  ,the  steepness  of  the  isobars,  is  called  a 
difference  in  the  intensity  of  the  weather.  Hence,  when 
we  speak  of  a  cyclone  as  being  intense,  we  mean  that  it 
has  steep  isobars  somewhere.  The  word  "  intensity  "  will 
occur  very  often  in  these  pages,  for  when  we  come  to  talk 
about  the  general  sequence  of  weather  from  day  to  day, 
we  shall  find  that  there  is  no  difference  betiueen  tlie  cyclones 
ivhich  cause  storms  and  those  ivhich  cause  ordinary  weather 
except  intensity.  This  is  another  of  the  fundamental 
principles  of  meteorology. 

Returning  now  to  our  cyclone,  the  whole  of  the  por- 
tion in  front  of  the  centre  facing  the  direction  towards 
which  it  moves  is  called  its  front,  and  the  whole  of  this 
portion  may  obviously  be  divided  into  a  right  and  left 
front.  The  other  side  of  the  centre  is,  of  course,  the  rear 
of  the  cyclone.  Then,  as  the  whole  cyclone  moves  along 
its  course,  it  is  evident  that  the  barometer  will  be  falling 
more  or  less  at  every  portion  of  the  front,  and  rising  more 
or  less  everywhere  in  rear,  so  that  there  must  be  a  line  of 
places  somewhere  across  the  cyclone,  where  the  barometer 
has  touched  its  lowest  point  and  is  just  going  to  rise. 


30  WEATHER. 

This  line  is  called  the  "  trough  "  of  the  cyclone,  because 
if  we  look  at  the  barometer-trace  at  any  one  place,  the 
"  ups  "  and  "  downs  "  suggest  the  analogy  of  waves,  so 
that  the  lowest  part  of  a  trace  may  be  called  a  "  trough." 
Or  we  may  look  at  the  cyclone  as  a  circular  eddy,  moving 
in  a  given  direction,  and  so  far  presenting  some  analogy 
to  a  wave.  Here  we  are  face  to  face  with  the  primary 
difficulty  of  understanding  synoptic  charts.  When  we 
look  at  any  chart  of  a  cyclone  which  represents  the  state 
of  things  existing  at  some  one  moment,  there  is  little  to 
suggest  the  idea  of  a  trough,  because  the  latter  depends 
on  the  motion  of  the  cyclone,  which  cannot  be  shown  on  a 
chart.  Perhaps  the  following  illustration  may  help  to 
explain  the  nature  of  the  trough.  Suppose  the  cyclone 
represented  the  inside  of  a  conical  crater,  if  we  walked 
along  the  line  that  marks  the  path  of  the  centre  from  the 
word  "  FRONT  "  to  the  word  "  REAR  "  on  the  diagram,  we 
should  pass  over  the  centre  of  the  crater,  and  be  walking 
downhill  all  the  time  till  we  reached  the  bottom,  and  up- 
hill afterwards.  But  now,  if  we  walked  across  the  crater 
on  any  other  line  parallel  to  this  one,  say  from  the  word 
"  pale  "  to  the  word  "  COOL,"  we  shall  equally  walk  down- 
hill till  we  arrive  at  the  point  occupied  by  the  letter  q  in 
the  word  "squalls."  At  q  we  should  still  be  on  the  side 
of  the  crater,  and  some  distance  from  the  centre,  but 
after  passing  q  we  should  begin  to  walk  uphill. 

WThen  we  have  once  realized  the  meaning  of  the 
trough,  we  shall  never  fall  into  the  very  common  error  of 
thinking  that  because  our  barometer  has  begun  to  rise, 
the  centre  of  the  cyclone  has  necessarily  passed  over 
us.  It  is  probably  only  the  trough,  but  we  will  explain 


WEATHER-PROGNOSTICS.  31 

afterwards  how  we  can  tell  whether  it  is  the  centre  or 
not. 

So  far  for  the  shape  and  names  of  the  different  por- 
tions of  the  cyclone.  Now  for  the  wind.  A  glance  at  the 
arrows  will  show  that,  broadly  speaking,  the  wind  rotates 
round  the  centre  in  a  direction  opposite  to  the  motion  of 
the  hands  of  a  watch.  That  is  to  say,  that  in  the  extreme 
front,  following  the  outer  isobar,  the  wind  is  from  the 
south-east ;  further  round,  it  is  from  the  east-north-east ; 
still  further,  from  the  north-north-west ;  then  from  about 
west ;  and,  finally,  from  the  south-west.  Then  we  note 
that  in  front  the  wind  is  slightly  incurved  towards  the 
centre,  and  therefore  blows  somewhat  across  the  isobars, 
while  in  rear  it  has  little  or  no  incurvature,  and  blows 
nearly  parallel  to  the  isobars.  The  velocity  or  force  of 
the  wind  will  depend  on  the  closeness  of  the  isobars.  In 
the  diagram  they  are  much  closer  set  in  rear  than  in 
front  of  the  cyclone,  and  therefore  the  wind  is  strongest 
behind  the  centre. 

In  our  chapter  on  clouds  we  shall  have  to  go  much 
more  minutely  into  the  nature  of  wind,  both  on  the  sur- 
face and  in  the  upper  currents ;  but  here  we  wish  to  confine 
our  attention  as  far  as  possible  to  the  weather  and 
appearance  of  the  sky. 

For  the  same  reason,  the  details  of  gradients  will  not 
be  developed  till  we  come  to  our  chapter  on  wind  and 
calms. 

The  weather  in  a  cyclone  is  somewhat  complicated. 
Some  characteristic  features  depend  on  the  position  of 
the  trough,  and  have  nothing  to  do  with  the  centre.  For 
instance,  the  weather  and  sky  over  the  whole  front  of  the 


32  WEATHER. 

cyclone — that  is,  all  that  lies  in  front  of  the  trough — is- 
characterized  by  a  muggy,  oppressive  feel  of  the  air,  and 
a  dirty,  gloomy  sky  of  a  stratiform  type,  whether  it  is 
actually  raining  or  only  cloudy.  On  the  other  side,  the 
whole  of  the  rear  is  characterized  by  a  sharp,  brisk  feel  of 
the  air,  and  a  hard,  firm  sky  of  cumulus  type. 

But,  on  the  contrary,  other  characteristic  features  are 
related  to  the  centre,  and  have  little  to  do  with  the 
trough.  The  rotation  of  the  wind,  though  slightly 
modified  near  the  trough,  is  in  the  main  related  to  the 
centre,  and  the  broad  features  of  the  weather  in  a  cyclone 
are — a  patch  of  rain  near  the  centre,  a  ring  of  cloud  sur- 
rounding the  rain,  and  blue  sky  outside  the  whole  system. 
The  centre  of  the  rain-area  is  rarely  concentric  with  the 
isobars.  It  usually  extends  further  in  front  thaji  in  rear, 
and  more  to  the  south  than  to  the  north,  but  is  still 
primarily  related  to  the  centre. 

This  will  be  readily  seen  by  reference  to  the  diagram  ; 
there  the  drizzle  and  driving  rain  extend  some  distance  to 
the  right  front,  while  almost  directly  behind  the  centre 
patches  of  blue  sky  become  visible.  Thus  a  cyclone  has, 
as  it  were,  a  double  symmetry :  that  is  to  say,  one  set  of 
phenomena,  such  as  warmth,  cloud  character,  etc.,  which 
are  symmetrically  disposed  in  front  and  rear  of  the  trough  ; 
and  another  set,  such  as  wind  and  rain,  which  are  sym- 
metrically arranged  round  the  centre.  There  is  reason  to 
believe  that  what  we  may  call  the  circular  symmetry  of  a 
cyclone  is  due  to  the  rotation  of  the  air,  while  the  pro- 
perties which  are  related  to  the  trough  are  due  to  the 
forward  motion  of  the  whole  system. 

As  this  is  a  somewhat  difficult  conception,  perhaps 


WEATHER-PKOGNOSTICS.  33 

the  following  analogy  may  not  be  out  of  place.  Let  us 
consider  the  twofold  distribution  of  the  population  of 
London.  As  regards  density,  we  find  a  comparatively  thinly 
populated  district  in  the  centre  of  London — that  is,  in  the 
City  proper.  Bound  this  there  is  a  tolerably  symmetrical 
ring  of  very  densely  populated  streets,  outside  of  which 
the  population  thins  away  towards  the  suburbs.  But  at 
the  same  time  London  is  divided  into  very  well-defined 
halves  of  comparative  poverty  and  wealth — the  east  and 
west  ends  respectively.  This  is  a  far  more  strongly 
marked  distinction  than  any  which  is  found  between  the 
north  and  south  sides  of  London,  in  spite  of  a  river  that 
might  have  been  supposed  to  make  a  natural  boundary. 
This  distinction  into  an  east  and  west  end  is  always  attri- 
buted to  the  general  march  of  the  population  westwards. 
Thus  the  front  and  rear  of  the  moving  population  have  a 
symmetry  independent  of  the  density  of  the  population 
round  a  centre. 

Returning  now  to  the  details  of  weather  in  a  cyclone, 
we  have  marked  on  the  diagram  the  kind  of  weather 
and  cloud  which  would  be  found  in  different  parts  of 
a  cyclone.  The  first  thing  which  will  strike  us  is  that 
the  descriptive  epithets  applied  to  the  sky  contain  the 
phraseology  of  the  most  familiar  prognostics.  At  the 
extreme  front  we  see  marked  "  pale  moon,"  "  watery  sun/' 
which  means  that  in  that  portion  of  a  cyclone  the  moon 
or  sun  will  look  pale  or  watery  through  a  peculiar  kind  of 
sky.  But  all  over  the  world  a  pale  moon  and  watery  sun 
are  known  as  prognostics  of  rain.  Why  are  they  so  ? 
The  reason  we  can  now  explain.  Since  a  cyclone  is 
usually  moving,  after  the  front  part  where  the  sky  gives 


34  WEATHER. 

a  watery  look  to  the  sun  has  passed  over  the  observer,  the 
rainy  portion  will  also  have  to  come  over  him  before  he 
experiences  the  blue  sky  on  the  other  side  of  the  cyclone. 

Suppose  the  cyclone  stood  still  for  a  week,  then  the 
observer  would  see  a  watery  sky  for  a  week,  without  any 
rain  following.  Suppose  the  cyclone  came  on  so  far  as  to 
bring  him  under  a  watery  sky,  and  then  died  out  or 
moved  in  another  direction,  then,  after  seeing  a  watery 
sky,  no  rain  would  fall,  but  the  sky  would  clear.  The 
prognostic  would  then  be  said  to  fail,  but  the  word  is  only 
partially  applicable.  The  watery  sky  was  formed  and  seen 
by  the  observer,  because  he  was  in  the  appropriate  portion 
of  the  cyclone,  and  so  far  the  prognostic  told  its  story 
correctly — viz.  that  the  observer  was  in  the  front  of  the 
rainy  area  of  a  cyclone.  The  prognostic  failed  in  its 
ordinary  indication  because  the  cyclone  did  not  move  on 
as  usual,  but  died  out,  and  therefore  never  brought  its 
rainy  portion  over  the  observer.  This  is  the  commonest 
source  of  the  so-called  failure  of  a  rain-prognostic  in 
Great  Britain.  The  reason  why  all  rain  is  not  preceded 
by  a  watery  sky  is  because  there  are  other  sources  of  rain 
besides  a  cyclone,  which  are  preceded  by  a  different  set  of 
weather-signs.  Such  is  the  whole  theory  of  prognostics. 

The  same  reasoning  which  applies  to  a  watery  sky 
holds  good  for  every  other  cyclone-prognostic.  We  shall 
have  explained  why  any  prognostic  portends  rain  when 
we  have  shown  that  the  kind  of  sky  or  other  appearance 
which  forms  the  prognostic  belongs  to  the  front  of  the  rainy 
portion  of  a  cyclone.  Conversely  we  shall  have  explained 
why  any  prognostic  indicates  finer  weather  when  we  have 
shown  that  the  kind  of  sky  belongs  to  the  rear  of  a 


WEATHER-PROGNOSTICS.  35 

Cyclone.  It  will  be  convenient,  therefore,  to  describe  the 
weather  in  different  parts  of  a  cyclone,  and  the  appro- 
priate prognostics  together. 

First,  to  take  those  prognostics  which  depend  on 
qualities  common  to  the  whole  front  of  the  cyclone,  viz. 
a  falling  barometer,  increased  warmth  and  damp,  with 
a,  muggy,  uncomfortable  feel  of  the  air,  and  a  dirty  sky. 

From  the  increasing  damp  in  this  part  of  a  cyclone, 
while  the  sky  generally  is  pretty  clear,  cloud  forms  round 
and  "caps"  the  tops  of  hills,  which  has  given  rise  to 
numerous  local  sayings*  The  reason  is  that  a  hill  always 
deflects  the  air  upwards.  Usually  the  cold  caused  by 
ascension  and  consequent  expansion  is  not  sufficient  to 
lower  the  temperature  of  the  air  below  the  dew-point ; 
but  when  very  damp,  the  same  amount  of  cooling  will 
bring  the  air  below  the  dew-point,  and  so  produce  con- 
densation. 

From  the  same  excessive  damp  the  following  may  be 
explained : — 

"When  walls  are  more  than  usually  damp,  rain  is 
expected." 

The  Zuni  Indians  in  New  Mexico  say  that  "  When  the 
locks  of  the  ISTavajos  grow  damp  in  the  scalp-house,  surely 
it  will  rain."  From  this  we  may  assume  that  scalps  are 
slightly  hygroscopic,  probably  from  the  salt  which  they 
contain. 

Also,  owing  to  excessive  moisture,  clouds  appear  soft 
and  lowering,  and  reflect  the  glare  of  ironworks  and  the 
lights  of  large  towns. 

With  the  gloomy,  close,  and  muggy  weather,  some 
people  are  troubled  with  rheumatic  pains  and  neuralgia, 


36  WEATHER. 

old  wounds  and  corns  are  painful,  animals  and  birds  are 
restless,  and  drains  and  ditches  give  out  an  offensive 
smell. 

A  glance  at  the  diagram  will  show  that  the  barometer 
falls  during  the  whole  of  the  front  of  the  cyclone.  There- 
fore the  explanation  of  the  universally  known  fact  that 
the  barometer  generally  falls  for  bad  weather  is,  that  both 
rain  and  wind  are  usually  associated  with  the  front  of  a 
cyclone.  When  we  discuss  secondaries,  we  shall  find 
a  kind  of  rain  for  which  the  barometer  does  not  fall ;  and 
in  our  chapter  on  forecasting  for  solitary  observers  we 
shall  explain  why  it  sometimes  rains  while  the  barometer- 
is  rising,  and  why  there  is  sometimes  fine  weather  while 
the  mercury  is  falling. 

Now  to  take  prognostics  which  belong  to  different 
portions  of  the  cyclone-front. 

By  reference  to  Fig.  2  it  will  be  seen  that  in  the  out- 
skirts of  the  cyclone-front  there  is  a  narrow  ring  of  halo- 
forming  sky.  Hence  the  sayings — 

"  Halos  predict  a  storm  (rain  and  wind,  or  snow  and 
wind)  at  no  great  distance,  and  the  open  side  of  the  halo 
tells  the  quarter  from  which  it  may  be  expected." 

"  Mock  suns  predict  a  more  remote  and  less  certain 
change  of  weather." 

With  regard  to  the  open  side  of  the  halo  indicating 
the  quarter  from  which  the  storm  may  be  expected,  it 
does  not  appear  that  this  can  be  used  as  a  prognostic  with 
any  certainty.  It,  however,  most  probably  originated  in 
the  fact  that  halos  are  usually  seen  in  the  south-west 
or  west,  when  the  sun  or  moon  is  rather  low,  the  lower 
portion  of  the  halo  being  cut  off  by  the  gloom  on  the 


WEATHER-PROGNOSTICS.  37 

horizon,  and  that  European  storms  generally  come  from 
those  quarters :  a  heavy  bank  of  cloud  will  often  lie  in 
that  direction. 

Inside  the  halo  sky  comes  the  denser  cloud  which  gives 
the  pale  watery  sun  and  moon.  Still  nearer  the  centre  we 
find  rain,  first  in  the  form  of  drizzle,  then  as  driving  rain. 
In  the  left  front  we  find  ill-defined  showers  and  a  dirty  sky. 

We  have  now  come  to  the  trough  of  the  cyclone.  The 
line  of  the  trough  is  often  associated  with  a  squall  or 
heavy  shower,  commonly  known  as  "  a  clearing  shower." 
This  is  much  more  marked  in  the  portion  of  the  trough 
which  lies  to  the  south  of  the  cyclone's  centre  than  on 
the  northern  side. 

Then  we  enter  the  rear  of  the  cyclone.  The  whole  of 
the  rear  is  characterized  by  a  cool,  dry  air,  with  a  brisk, 
exhilarating  feel,  and  a  bright  sky,  with  hard  cumulus 
cloud.  These  features  are  the  exact  converse  of  those  we 
found  in  the  cyclone-front.  In  the  cloud-forms  especially 
we  see  this  difference.  All  over  the  front,  whether  high 
up  or  low  down,  whether  as  delicate  cirrus  or  heavy 
gloom,  the  clouds  are  of  a  stratified  type.  Even  under 
the  rain,  when  we  get  a  peep  through  a  break  in  the 
clouds,  we  find  them  lying  like  a  more  or  less  thick  sheet 
over  the  earth.  All  over  the  rear,  on  the  contrary,  clouds 
take  the  rocky  form  known  as  cumulus  ;  cirrus  is  almost 
unknown  in  the  rear  of  a  cyclone-centre  in  the  temperate 
zone. 

In  the  exhilarating  quality  of  the  air  we  find  the 
meaning  of  the  proverb — 

"  Do  business  with  men  when  the  wind  is  in  the  north- 
west." 


38  WEATHER. 

A  north-west  wind  belongs  to  the  rear  of  a  cyclone, 
and  improves  men's  tempers,  as  opposed  to  the  neuralgic 
and  rheumatic  sensations  in  front  of  a  cyclone,  which 
make  them  cross. 

As  to  the  details  of  the  different  portions  of  the  rear. 
Immediately  behind  the  centre  small  patches  of  blue  sky 
appear.  Further  from  the  centre  we  find  showers  or 
cold  squalls ;  beyond  them,  hard  detached  cumulus  or 
strato-cumulus  ;  still  farther  the  sky  is  blue  again. 

In  the  south  of  the  cyclone,  near  the  outskirts,  the 
long  wispy  clouds  known  as  windy  cirrus  and  "mares' 
tails  "  are  observed.  These  indicate  wind  rather  than  rain, 
as  they  are  outside  of  the  rainy  portion  of  the  cyclone. 

So  far  we  have  only  described  the  different  kinds 
of  weather  which  would  be  experienced  at  the  same 
moment  in  different  places.  We  have  not  said  much 
about  the  sequence  of  weather  at  any  one  place.  A  single 
chart  tells  little  about  this,  for  it  does  not  indicate  which 
way  the  cyclone  is  going.  To  track  a  cyclone  we 
want  another  chart  about  twelve  or  twenty-four  hours 
later,  from  which  we  can  see  exactly  how  the  cyclone- 
centre  has  moved.  Then  we  can  follow  the  sequence  of 
weather  for  those  twelve  or  twenty-four  hours  at  any  place 
we  choose  to  select. 

It  must  specially  be  borne  in  mind  that  the  word 
"front"  is  a  relative  term.  In  our  diagram  we  have 
pointed  it  to  the  north-east,  because  that  is  the  direction 
towards  which  the  majority  of  British  cyclones  move. 
In  very  rare  cases  we  get  a  cyclone  moving  from  the 
south-east.  The  general  circulation  of  wind  then  remains 
about  the  same,  but  the  characteristic  qualities  of  the 


WEATHER-PROGNOSTICS.  39 

different  portions  of  a  cyclone  are  shifted  to  the  new 
position  of  the  front  and  rear.  For  instance,  if  the  cyclone 
in  our  diagram  was  moving  towards  the  north-west,  we 
should  have  muggy  weather  and  dirty  sky  with  a  north- 
west wind,  and  bright  weather  and  clear  sky  with  south- 
west wind.  This  occurs  habitually  in  the  northern  Tropics, 
but  very  rarely  in  temperate  regions. 

But  now  to  take  the  diagram  as  it  is  drawn.  We  will 
suppose  that  the  centre  has  moved  along  the  dotted  line, 
towards  the  north-east,  till  it  is  outside  the  margin  of  the 
figure.  What  would  the  sequence  of  weather  be  to  an 
observer  who  was  living,  say,  where  the  word  "  halo  "  is 
written,  just  below  the  word  "  FRONT."  This  we  may  get 
by  taking  a  line  across  the  diagram,  parallel  to  the  line 
which  marks  the  track  of  the  cyclone.  This  will  take 
us  to  the  word  " detached"  just  below  the  word  "  REAR." 
Following  the  words  and  symbols,  we  should  find  that  as 
the  barometer  began  to  fall  a  halo-forming  sky  would 
appear,  with  the  wind  coming  light  from  the  south-east. 
Soon  the  sky  would  grow  lower  and  denser,  into  what  is 
known  as  a  "  watery  "  sky,  and  the  wind  would  begin  to 
veer  towards  the  south  and  to  come  in  uneasy  gusts.  Then 
drizzling  rain  would  set  in,  the  barometer  still  falling,  and 
the  muggy,  disagreeable  feel  of  the  air  would  be  very 
noticeable.  Later,  the  wind  would  begin  to  rise  from  the 
south-west,  driving  the  rain  before  it,  and  perhaps  attain 
the  force  of  a  gale.  After  a  time  one  of  the  gusts  would 
be  much  harder,  with  heavier  rain  than  any  which  had 
been  previously  experienced,  and  with  a  squall  the  wind 
would  go  round  with  a  jump  two  or  three  points  of  the 
compass  to  the  west  or  west-north-west.  If  we  looked  at 


40  WEATHER. 

the  barometer,  we  should  find  that  at  that  moment  the 
mercury  had  begun  to  rise ;  this  is  the  passage  of  the 
trough  of  the  cyclone.  The  wind  now  blows  harder  than 
it  had  done  before,  and  comes  in  squalls  from  the  north- 
west, while  the  whole  aspect  of  the  sky  and  character  of 
weather  are  changed.  The  air  is  cold  and  dry,  the  sky  is 
higher  and  harder,  some  patches  of  blue  appear  in  the 
heavens,  hard  rocky  cumulus  appears  on  the  top  of  the 
squalls  or  showers,  and  the  wind  moderates.  Gradually 
showers  are  replaced  by  masses  of  cloud  from  which  no 
rain  descends,  and  after  a  time  the  sky  becomes  bright 
and  cloudless,  while  the  wind  falls  to  a  gentle  breeze. 

We  have  endeavoured  to  show  all  this  in  a  diagram- 
matic form  in  Fig.  3  j  but  observe  that,  while  we  read  the 


^g.o 


\  if'* 

I  Trough, 

Halo.    -     "Watery.      Drizzk.          Rain.  Squall.      Showers.    Crm*  .'Blue. 
FIG.  3. — Weather  sequence  in  a  cyclone. 

other  diagram  from  right  to  left,  this  one  we  read  in  the 
ordinary  manner  from  left  to  right.  This  inversion  is 
obviously  necessary,  because  the  cyclone  is  moving  from 
ri^ht  to  left.  The  upper  line  gives  the  trace  which  a 


WEATHER-PROGNOSTICS.  41 

self-recording  barometer  would  have  marked.  In  front 
of  the  cyclone,  where  the  gradients  are  moderate,  the 
mercury  falls  slowly;  in  rear,  where  the  gradients  are 
steep,  it  rises  rapidly.  The  arrows  below,  which  are 
supposed  to  fly  with  the  wind,  marks  the  shift  of  wind 
which  an  observer  would  experience ;  and  the  number  of 
barbs  denote  the  varying  force  of  the  wind.  The  sequence 
of  weather,  which  is  written  in  words,  is  identical  with 
the  sequence  of  weather  as  marked  on  the  plan  of 
cyclone-prognostics. 

Wind  is  said  to  "  veer,"  or  "  haul,"  when  it  changes 
in  the  same  direction  as  the  course  of  the  sun ;  that  is, 
from  east,  by  south,  to  west,  or  from  west,  by  north, 
round  to  east  again.  Wind  is  said,  on  the  contrary,  to 
"  back  "  when  it  changes  against  the  sun ;  that  is,  from 
east,  by  north,  to  west,  and  from  west,  by  south,  to  east 
again.  We  have  seen  that  the  wind  veers  to  an  observer 
situated  to  the  southward  of  a  cyclone-centre.  An 
inspection  of  Fig.  2  will  show  that  the  wind  would  back 
from  east  to  north-east,  and  then  through  north  to  north- 
west, if  the  observer  was  situated  anywhere  north  of  the 
cyclone's  path.  If  he  was  exactly  on  the  path  of  the 
centre,  the  wind  would  jump  round  from  south-west  to 
north-east  without  either  veering  or  backing;  so  that  by 
watching  the  wind  any  one  can  tell  what  part  of  a  cyclone 
he  is  in.  In  Northern  Europe  cyclones  rarely  pass  so  far 
to  the  south  as  to  give  the  backing  sequence.  When 
they  do  they  are  almost  always  soon  followed  by  another 
cyclone,  which  passes  farther  north,  and  brings  fresh  bad 
weather,  with  another  nearly  complete  shift  of  wind. 
Hence  the  meaning  of  the  following  prognostic : — 


42  WEATHER. 

"  When  the  wind  veers  against  the  sun, 
Trust  it  not,  for  back  'twill  run." 

Here  "  veering  "  is  used  for  shifting  generally,  and  not 
in  its  more  limited  sense  of  shifting  in  one  particular  way. 

The  explanation  which  we  have  just  given  as  to  the 
squall  which  occurs  after  the  barometer  has  turned  in  a 
cyclone,  or  at  the  trough  of  the  cyclone,  will  show  the 
truth  of  the  following :  — 

"  When  rise  begins,  after  low, 
Squalls  expect  and  clear  blow." 

The  clear  blow  refers  to  the  brighter  gale  which  comes 
from  the  north-west,  as  contrasted  with  the  dirty  gale 
which  preceded  it  from  the  south-west. 

If  we  take  a  general  view  of  all  the  weather  in  a 
cyclone,  we  find  that  we  have  a  large  number  of  prog- 
nostics which  precede  the  rain  that  is  associated  with 
wind  and  a  falling  barometer.  We  will  now  introduce 
our  readers  to  a  kind  of  rain  which  is  associated  with 
calm  and  a  stationary  barometer. 

SECONDARY  CYCLONES. 

A  secondary  cyclone,  or,  shortly,  a  "  secondary,"  is  so 
called  both  because  it  has  some  features  in  common  with 
a  primary  cyclone,  and  because  its  origin  and  motion  are 
frequently  determined  by  the  path  of  the  primary.  It  is, 
however,  often  found  at  the  edges  of  anticyclones  without 
any  primary,  and  in  many  parts  of  the  world  it  is  of 
frequent  occurrence  where  primaries  are  almost]  unknown. 
The  general  appearance  of  that  shape  of  isobars  to  which 
we  attach  the  name  of  secondary  will  be  readily  seen  by 


WEATHER-PROGNOSTICS.  43 

an  inspection  of  Fig.  4.  In  that  diagram  the  general 
slope  of  the  isobars  is  towards  the  north,  but  the  isobar 
of  30*1  ins.  (765  mm.)  is  bent  into  a  loop,  so  as  to 
enclose  an  area  of  relatively  lower  pressure,  but  not  a 
regular  pit  of  low  pressure  as  in  a  primary  cyclone.  In 
consequence  of  this  the  isobaric  slope  is  diminished,  as 
the  distance  between  the  two  adjacent  isobars  is  increased. 


FIG.  4. — Weather  in  secondary  cyclone. 

For  the  same  reason,  the  wind  inside  the  loop  is  very 
light,  but  round  the  edges  of  the  loop  the  gradient  is 
increased,  and  the  wind  is  stronger.  This  wind  is  usually 
in  angry,  violent  gusts,  and  not  in  the  steady,  heavy  blow 
of  cyclone-wind. 

The  motion  of  the  secondary  is  usually  parallel  to  the 
path  of  the  primary,  and  very  rarely  shows  any  tendency 
to  revolve  round  the  primary.  Hence  the  term  "  satellite  " 


44  WEATHEE. 

depressions,  which  is  sometimes  applied  to  secondaries, 
seems  hardly  suitable. 

When  a  secondary  is  formed  at  the  edge  of  an 
anticyclone,  the  motion  is  generally  very  obscure. 

Some  very  striking  weather-changes  are  grouped 
round  this  loop  of  low  pressure.  In  the  extreme  front  we 
find  the  thin  nebulous  cloud  which  forms  halo.  Beyond 
this  is  a  narrow  ring  of  gloomy  cirro-stratus;  then  we 
find  a  ring  of  heavy  rain,  with  gusts,  surrounding  that  half 
of  the  secondary  where  the  gradients  are  steepest.  Inside 
this,  in  the  heart  of  the  secondary,  we  find  a  calm,  with  a 
steady,  heavy  downpour  of  rain.  On  the  rear  side  of  the 
ring  of  gusts  there  is  a  narrow  belt  of  irregular  cumulus, 
beyond  which  the  sky  is  blue.  On  the  low-pressure  side 
of  the  secondary  we  find  cirrus  and  cirro-stratus,  and 
outside  that  the  cloud  appropriate  to  the  primary  cyclone. 
The  general  course  of  the  wind  follows  the  universal  laws 
of  wind  and  gradient.  The  arrows  in  the  diagram  show 
that  the  direction  of  the  wind  would  be  with  the  slope  of 
gradients  which  we  have  drawn  there.  We  may  note 
that  the  amount  of  deflection  of  the  wind  is  much  smaller 
than  in  primary  cyclones.  All  over  the  world  secondaries 
are  associated  with  a  peculiar  class  of  thunderstorm,  but 
we  are  unable  to  say  in  what  particular  portion  they  are 
localized. 

If  the  primary  was  moving  in  the  direction  of  the 
dotted  arrow  marked  "FKONT"  on  the  diagram,  the 
sequence  of  weather  to  a  single  observer  would  be  as 
follows.  The  blue  sky  would  become  covered  either  with 
a  thin  nebulous  haze,  and  perhaps  halos,  or  else  with 
dirty  cirro-stratus,  and  the  wind]  would  fall  light.  The 


WEATHER-PROGNOSTICS.  45 

clouds  would  rapidly  become  black  and  heavy,  and  soon, 
with  some  angry  gusts,  heavy  rain  with  big  drops  would 
commence  suddenly.  We  saw  before  that  in  a  primary  the 
rain  begins  as  drizzle.  If  the  barometer  is  very  carefully 
watched,  a  very  slight  rise  or  fail  will  occur  now ;  perhaps 
only  one-hundredth  of  an  inch.  This  gusty  rain  only  lasts 
a  few  minutes,  when  the  wind  falls,  and  the  rain  pours 
straight  down,  not  quite  as  heavily  as  at  first.  This  stage 
sometimes  lasts  four  or  five  hours,  and  is  often  very  puzzling 
to  Englishmen  and  others  who  are  accustomed  to  associate 
rain  with  a  falling,  and  not  with  a  steady  barometer. 
When  the  rear  edge  of  the  secondary  approaches,  the  rain 
suddenly  becomes  much  heavier,  with  more  angry  gusts. 
Just  at  this  moment  the  barometer  moves  slightly — 
maybe  not  more  than  one-hundredth  of  an  inch.  If  the 
general  motion  of  the  mercury  was  upwards  when  the 
rain  began,  this  second  motion  will  be  upwards  also ;  if 
downwards,  this  will  be  downwards  also.  Here  is  another 
contrast  to  a  primary  cyclone,  where  a  fall  of  the  mercury 
is  always  followed  by  a  rise.  The  heavy  rain  lasts  a  very 
short  time,  when  the  clouds  break  quickly  into  irregular 
cumulus,  and  the  sky  is  soon  clear  again. 

We  have  endeavoured  to  show  this  in  the  annexed 
diagram  (Fig.  5).  The  upper  curve  shows  the  barometric 
changes  to  a  single  observer.  In  the  secondary  of  which 
we  have  drawn  the  plan  in  Fig.  4,  the  general  motion  of 
the  barometer  would  be  downwards ;  so  in  Fig.  5  we  find 
that  the  small  motion  of  the  barometer  as  the  secondary 
approaches  is  downwards.  Then  the  mercury  remains 
perfectly  steady  till  the  disturbance  is  just  going  to  pass 
off,  when  it  takes  another  small  step  downwards,  and  then- 


46  WEATHER. 

continues  to  fall  slowly,  as  before  the  rain  began.  Below 
the  barogram,  the  sequence  of  weather  is  shown  in  a 
diagrammatic  form,  so  as  to  show  clearly  the  ring-like 
character  of  the  rain.  The  lowest  bar,  marked  overcast, 
is  drawn  under  the  barogram  during  the  whole  time  that 
the  sky  was  overcast.  The  upper  shaded  bar  is  drawn 
double  thickness  under  the  portions  of  the  barogram 

o  I  2  Hours 


^--—  ___^           Bar- 

_ 

Gusty 
Steady 
Overcast 

r 
r 

>^ 

1            ,-'              3 

t<X\^-v'v'\                          "     "                            '     \ 
1       v-:v-'                          '                                                'I 

FIG.  5. — Weather  sequence  in  secondary. 

during  which  the  heaviest  gusty  rain  was  experienced,  and 
single  thickness  during  the  time  that  the  steady  downpour 
was  observed. 

We  have  already  alluded  to  the  idea  of  intensity  in 
any  form  of  atmospheric  circulation ;  and  as  simple  cloud, 
moderate,  and  heavy  rain  are,  as  it  were,  three  successive 
degrees  of  intensity,  the  thickness  of  the  bars  in  the 
above  diagram  is  proportional  to  the  intensity  of  the 
weather  in  the  different  portions  of  a  secondary. 

There  are  no  special  prognostics  associated  with 
secondaries.  Our  object  in  mentioning  the  subject  here 
was  to  explain  the  nature  of  another  kind  of  rain  than 
that  which  is  found  in  primary  cyclones.  When  we 
come  to  explain  various  groups  of  prognostics,  we  shall 
find  a  number  of  rain-prognostics  which  depend  on  a 


.      WEATHER-PROGNOSTICS.  47 

diminution  of  barometric  pressure.  All  these  indications 
must  obviously  be  absent  before  the  rain  which  is  pro- 
duced by  a  secondary,  and  we  shall  now  understand  why 
rain  often  falls  without  these  pressure-prognostics  being 
observed. 


ANTICYCLONE-PROGNOSTICS. 

An  anticyclone  is  an  area  of  high  pressure  sur- 
rounded by  nearly  circular  isobars.  These  are  always  a 
considerable  distance  apart,  and  extend  over  a  large  area. 
The  pressure  is  highest  in  the  centre,  and  gradually 
diminishes  outwards.  The  air  is  calm  and  cold  in  the 
central  portion,  while  on  the  outskirts  the  wind  blows 
round  the  centre  in  the  direction  of  the  hands  of  a 
watch,  not  exactly  parallel  to  the  isobars,  but  spirally 
outwards.  Unlike  a  cyclone,  which  is  commonly  in  rapid 
motion,  an  anticyclone  is  often  stationary  for  many  days 
together. 

Thus  in  Fig.  6  we  see  that,  while  there  is  a  dead  calm 
in  the  centre,  the  wind  comes  from  the  north  on  the 
eastern  side,  from  north-east  on  the  south  side,  from 
south-east  on  the  western  edge,  and  from  west  on  the 
northern  edge. 

The  broad  features  of  the  weather  in  an  anticyclone 
are  blue  sky,  dry  cold  air,  a  hot  sun,  and  hazy  horizon, 
with  very  little  wind — in  fact,  the  very  antithesis  of 
everything  which  characterizes  a  cyclone.  As  a  neces- 
sary consequence  of  this,  we  find  in  an  anticyclone 
strongly  marked  radiation  weather,  and  much  diurnal 
variation.  These  two  last  ideas  are  so  important  that  we 


48 


WEATHER. 


must  devote  a  few  paragraphs  to  their  explanation  and 
consideration. 

Eadiation  weather  is  best  explained  by  an  example. 
On  a  very  fine  summer  day  we  generally  find  the  valleys 
full  of  mist  in  the  early  morning.  As  the  sun  gains 
power  the  vapour  rises  and  evaporates,  so  that  the  sky 


Cloudless 


Stratus 
Sometimes 


Bitter  E  Winds 
Black  Sly  in  Winter 


FIG.  6. — Anticyclone  prognostics. 

becomes  cloudless  and  the  sun  very  hot.  After  sunset, 
the  air  being  still  and  dry,  radiation  into  the  cold  space 
which  surrounds  the  earth  proceeds  rapidly,  and  soon 
mist  forms  again  in  the  hollows,  and  dew  upon  the 
grass. 


WEATHER-PROGNOSTICS.  49 

The  sequence  of  a  fine  day  in  winter  would  be  similar 
in  general  character,  but  differ  somewhat  in  details. 

Thus,  though  we  have  written  "fog,"  "light  cumulus," 
"  cloudless,"  in  certain  poitions  of  our  anticyclone  "dia- 
gram," these  words  only  describe  the  prominent  daytime 
weather  which  most  affects  us  in  the  various  portions  of 
the  anticyclone;  but  the  term  "RADIATION  WEATHER," 
written  in  capital  letters,  denotes  accurately  the  character 
by  night  and  by  day,  in  summer  or  in  winter. 

Moreover,  radiation  is  only  a  secondary  product  of  an 
anticyclone.  The  primary  feature  is  calm ;  radiation 
follows  as  a  matter  of  course,  and  the  weather  due  to 
radiation  varies  enormously  in  different  latitudes,  in 
different  seasons,  and  in  -different  localities  in  the  same 
country. 

The  theory  of  radiation  is,  that  when  the  air  is  still 
the  heated  surface  of  the  earth  radiates  into  the  cold 
surrounding  space,  and  so  the  former  grows  cold  enough 
to  condense  vapour  in  the  air  near  the  ground,  or  dew  on 
a  suitable  surface,  such  as  grass.  But  when  there  is  more 
or  less  wind,  each  successive  layer  of  air  which  is  in 
contact  with  the  radiating  earth  gets  removed  so  rapidly 
that  this  condensing  process  cannot  take  place,  and  then 
no  dew  or  fog  is  formed.  Thus  the  kind  and  amount  of 
cloud  is  variable,  but  always  dependent  on  radiation. 

On  the  contrary,  the  particular  kind  of  cloud  which 
forms  in  any  portion  of  a  cyclone  is  directly  and  primarily 
due  to  the  rising  current  induced  by  that  form  of 
rotating  air,  and  ve  shall  see  in  a  future  chapter  that 
the  day  character  of  these  clouds  is  not  altered  during 
the  night. 

E 


50  WEATHER. 

From  the  same  example  of  a  fine  day  we  can  readily 
pass  to  the  idea  of  diurnal  variation.  In  the  kind  of  day 
which  we  have  just  described  we  find  a  regular  sequence 
from  fog  to  blue,  and  back  again  to  fog,  following  the 
course  of  the  sun — that  is,  the  time  day. 

But  besides  the  amount  of  mist,  every  other  meteoro- 
logical element  has  a  complex  series  of  diurnal  changes 
which  depend  on  the  time  of  day.  For  instance,  both 
the  direction  and  velocity  of  the  wind  have  a  marked 
diurnal  period,  and  so  has  the  amount  of  cloud,  the 
amount  of  vapour,  and  every  other  component  of  weather. 
These,  and  many  others,  will  form  the  subject  of  a  sub- 
sequent long  and  rather  difficult  chapter.  All  that  we 
have  to  note  here  is  the  important  principle  of  meteorology 
— that  the  primary  character  of  all  weather  is  given  by 
the  shape  of  the  isobars,  whether  cyclonic,  anticyclonic,  or 
otherwise ;  that  a  complex  series  of  diurnal  changes  are 
superimposed  on  this,  which  modify,  but  do  not  alter  the 
intrinsic  quality ;  and  that  the  resulting  weather  is  the 
sum  of  the  two  together.  Thus  in  a  cyclone  the  changes 
of  weather  due  to  its  motion  are  so  marked  and  so  strong 
that  diurnal  changes  are  often  entirely  obliterated ;  while 
in  a  calm  anticyclone,  where  there  is  no  motion,  the 
uniform  character  of  the  general  weather  allows  full  play 
to  radiation,  and  the  diurnal  changes  are  very  prominent. 

Throughout  this  work  we  shall  call  the  character  and 
changes  of  weather  which  are  due  to  the  shapes  of  the 
isobars,  the  general  character  and  general  changes, 
because  they  are  caused  by  alterations  in  the  general 
distribution  of  pressure  over  a  large  portion  of  the  earth's 
surface.  On  the  contrary,  changes  which  are  due  to  tho 


WEATHER-PROGNOSTICS.  51 

time  of  day,  the  season  of  the  year,  or  to  any  local 
peculiarity,  we  shall  call  diurnal,  seasonal,  or  local  varia- 
tions of  the  general  character.  The  first  are  really 
changes,  the  second  only  variations.  The  reason  why 
many  prognostics  which  are  due  to  radiation  and  diurnal 
causes  are  signs  of  settled  fine  weather,  is  because  in  a 
country  like  England  they  can  only  occur  in  an  anti- 
cyclone. An  anticyclone  means  settled  fine  weather,  not 
only  because  the  weather  at  any  moment  in  it  is  fine,  but 
because  it  is  usually  stationary,  and  so  there  is  nothing 
to  change  the  existing  conditions.  All  anticyclone  prog- 
nostics fail  when  the  anticyclone  breaks  up  suddenly,  or 
in  the  not  very  common  case  when  it  moves  onward  along 
a  definite  path. 

We  will  now  give  a  few  prognostics  due  to  the  varia- 
tions of  an  anticyclone  in  some  detail. 

The  sky  being  generally  clear  and  the  air  calm,  the 
temperature  is  high  in  the  day  and  low  at  night.  In 
summer  brilliant  sunshine  prevails  during  the  day,  and 
at  night  there  is  a  heavy  dew,  and,  in  low-lying  places 
mist. 

"  Heavy  dews  in  hot  weather  indicate  a  continuance  of 
fair  weather,  and  no  dew  after  a  hot  day  foretells  rain." 

"If  mists  rise  in  low  ground  and  soon  vanish,  expect 
fair  weather." 

Fine,  bright,  genial  weather  raises  the  spirits  and 
exerts  an  enlivening  influence  not  only  on  human  beings, 
but  also  on  animals,  birds,  insects,  etc.  Hence  the 
sayings — 

"When  sea-birds  fly  out  early  and  far  to  seaward, 
moderate  winds  and  fair  weather  may  be  expected." 


52  WEATHER. 

"  If  rooks  go  far  abroad,  it  will  be  fine." 

"Cranes  soaring  aloft  and  quietly  in  the  air  fore- 
shadows fair  weather." 

"  If  kites  fly  high,  fine  weather  is  at  hand." 

"  Bats  or  field-mice  coming  out  of  their  holes  quickly 
after  sunset  and  sporting  themselves  in  the  open  air, 
premonstrates  fair  and  calm  weather." 

"  Chickweed  expands  its  leaves  boldly  and  fully  when 
fine  weather  is  to  follow." 

These  are  merely  samples  of  innumerable  similar 
prognostics  in  all  parts  of  the  world. 

In  winter  frost  is  generally  prevalent  in  the  central 
area  of  an  anticyclone,  accompanied  frequently  by  fog, 
which  is  most  dense  in  the  neighbourhood  of  large  towns. 
This  is  all  due  to  the  radiation  of  calm  weather. 

"  White  mist  in  winter  indicates  frost." 

The  wind  is  usually  very  light  in  force. 

"It  is  said  to  be  a  sign  of  continued  good  weather 
when  the  wind  so  changes  during  the  day  as  to  follow  the 
sun." 

This  "veering  with  the  sun,"  as  it  is  called,  is  the 
ordinary  diurnal  variation  of  the  wind,  which  in  England 
is  only  very  obvious  with  the  shallow  gradients  of  an 
anticyclone.  At  seaside  places  in  summer  very  often 
"  the  wind  is  in  by  day  and  out  by  night,"  which  is  the 
equivalent  of  the  land  and  sea  breezes  of  the  tropics. 
Like  the  preceding  prognostic,  it  is  only  in  anticyclones 
that  local  currents  of  air,  probably  due  to  unequal  heating 
of  sea  and  land,  can  override  the  general  circulation  of 
the  atmosphere  in  this  country. 

Sometimes  in  winter,  on  the  southern  side  of  the  anti- 


WEATHER-PROGNOSTICS.  53 

cyclone,  bitter  east  winds  with  a  black-looking  sky  will 
prevail  for  several  days  together,  when  it  may  truly  be 
said — 

"  When  the  wind  is  in  the  east, 
It  is  neither  good  for  man  nor  beast." 

This  class  of  anticyclone  prognostics  hold  good  as 
long  as  the  anticyclone  remains  stationary.  Occasionally 
the  anticyclone  moves  on,  and  is  replaced  by  some  other 
form  of  isobars  ;  but  far  more  frequently  the  anticyclone 
breaks  up — that  is  to  say,  it  disappears  without  moving 
on,  and  is  replaced  by  a  cyclone  or  some  other  type  of 
isobars. 

WEDGE-SHAPED  ISOBARS. 

We  have  already  defined  wedge-shaped  isobars  as  a 
projecting  area  of  high  pressure  moving  along  between 
two  cyclones.  This  wedge  may  point  in  any  direction, 
but  in  practice  by  far  the  most  frequently  to  the  north. 
We  have  therefore  selected  such  a  one  for  the  diagram 
of  the  wind  and  weather  in  an  ideal  wedge,  which  we  give 
in  Fig.  7.  There  the  highest  pressure  is  at  the  bottom 
of  the  diagram,  while  the  wedge-shaped  isobars  project 
towards  the  north.  On  the  right  hand  we  see  the  rear  of 
a  retreating  cyclone ;  on  the  left,  the  front  of  an  advancing 
depression.  As  these  two  cyclones  move  forward,  the 
wedge  goes  on  between  them,  so  that  there  must  always 
be  a  line  of  stations  where,  after  the  barometer  has  risen 
owing  to  the  onward  passage  of  the  first  cyclone,  the 
mercury  has  just  begun  to  fall,  owing  to  the  advance 
of  the  second  depression.  This  line  is  called  the  crest 


WEATHER. 


of  the  wedge,  and  is  marked  by  a  dotted  line  in   the 
diagram. 

The  wind  blows  round  the  wedge  in  accordance  with 
the  universal  law  of  gradients.  Thus  on  the  east  side  of 
the  wedge  the  wind  is  from  north-west ;  in  the  centre  it 
is  calm ;  and  on  the  west  side,  from  south-west  to  south- 


\CREST 


Refraction 


Rain 


3o-2 


Blue 


FIG.  7. — Wedge-shaped  isobar  prognostics. 

east,  as  marked  by  the  symbols  on  the  diagram.  In 
practice  the  gradients  are  never  steep,  so  the  force  of  the 
wind  rarely  rises  to  above  that  of  a  pleasant  breeze. 

The  broad  features  of  the  cloud  and  weather  in  a 
wedge  are  written  across  the  diagram  (Fig.  7).     In  front 


WEATHER-PROGNOSTICS.  55 

we  find  blue  sky,  with  beautifully  fine  weather,  refraction, 
and  that  unusual  clearness  of  the  atmosphere  known  as 
"visibility."  Nearer  the  calm  under  the  crest  we  come  to 
radiation  weather,  with  fog;  and  then  to  halo-bearing 
sky  just  in  front  of  the  crest,  and  stripes  of  cirrus-cloud. 
A  thunderstorm  or  heavy  shower  is  often  experienced  at 
the  top  of  the  wedge.  In  rear  of  the  crest  the  sky 
becomes  covered  with  cirro-stratus  cloud,  and  further  in 
rear  we  find  the  rain  of  the  approaching  cyclone. 

Here,  as  in  cyclones,  we  see  the  striking  fact  that  the 
words  applied  to  describe  the  weather  contain  the  phrase- 
ology of  many  familiar  prognostics,  such  as  those  con- 
nected with  visibility,  halos,  or  the  stripes  of  cirrus  which 
form  the  cloud  popularly  known  as  "  Noah's  Ark." 

If  we  remember  how  we  took  a  sort  of  section  across  a 
cyclone,  and  so  found  the  sequence  of  weather  at  any 
station,  we  shall  readily  understand  that  we  have  only  to 
read  this  diagram  (Fig.  7)  from  right  to  left  to  get  the 
sequence  of  weather  during  the  passage  of  a  wedge. 
Thus  we  should  have  a  beautifully  fine  day,  with  a  north- 
west wind  and  a  rising  barometer,  with  a  hot  sun  by  day, 
and  a  cold  night  with  radiation  according  to  the  season 
of  the  year.  Then,  while  the  barometer  was  still  rising, 
the  blue  sky  would  assume  that  peculiar  nebulous  white- 
ness which  forms  halos,  and  stripes  of  cirrus  would  appear 
in  places.  Soon  the  barometer  would  begin  to  fall,  the 
sky  to  grow  denser  and  overcast,  and  before  long  the 
drizzling  rain  of  the  new  cyclone  would  begin  to  fall, 
the  wind  having  previously  backed  to  the  south-west. 

We  now  see  the  meaning  of  the  halo  and  cirrus-stripes 
being  marked  in  the  diagram  partly  in  front  of  the  crest 


56  WEATHER. 

of  the  wedge,  viz.  that  in  a  wedge  the  sky  shows  the 
approach  of  a  new  cyclone  before  the  barometer  at  a 
single  station  has  ceased  to  rise.  This  is  very  interesting, 
as  it  is  the  first  opportunity  we  have  had  of  explaining 
why  the  barometer  sometimes  appears  to  fail,  and  rises,  as 
in  this  case,  while  bad  weather  is  manifestly  approaching. 
It  also  shows  the  great  additional  power  of  forecasting 
which  the  use  of  synoptic  charts  gives  to  a  meteorologist, 
seated  in  a  central  office,  with  abundant  telegraphic  com- 
munication. Suppose  any  morning  that  a  forecaster  found 
his  isobars  were  wedge-shaped.  He  could  then  telegraph 
to  the  eastern  districts  of  his  territory  that  the  fine 
weather  would  not  last,  though  they  with  their  rising 
mercury  might  have  thought  the  contrary. 

It  may  be  remarked  here  that  all  cyclones  are  not 
preceded  by  a  wedge,  but  only  those  which  roll,  as  it 
were,  along  the  northern  edge  of  large  stationary  anti- 
cyclones. 

We  can  now  explain  in  detail  the  prognostics  that  are 
marked  on  the  diagram,  and  several  others  for  which 
there  was  no  room.  Any  appearance  of  the  sky  which 
characterizes  the  front  of  a  wedge  will  be  a  sign  of  rain, 
because  there  is  always  rain  in  rear  of  that  shape  of 
isobars.  These  prognostics  of  wet  which  are  associated 
with  fine,  dry  weather  are  particularly  interesting,  because 
they  are  the  very  opposite  of  the  rain-prognostics  in  a 
cyclone,  which  are  associated  with  increasing  damp  and 
a  dirty  sky. 

It  used  to  be  thought  that  every  prognostic  of  rain 
would  be  explained  by  showing  that  the  appearance  was 
due  to  an  increase  of  vapour  in  the  air ;  here  we  find  that 


WEATHEE-PROGNOSTICS.  57 

the  prognostic  can  only  be  explained  on  the  supposition 
that  many  cyclones  develop  an  area  of  calm  and  clear  sky 
in  front  of,  and  as  a  portion  of,  themselves.  It  is  a  crude 
idea  of  meteorology  to  think  that  all  rain-prevision  depends 
on  hygrometry. 

Keturning  now  to  Fig.  7,  we  see  that  in  the  rear  of 
the  retreating  cyclone  the  air  is  dry  and  the  weather 
beautifully  fine — of  the  sort  of  which  we  would  say  that  it 
was  "  too  fine  to  last ; "  or,  if  it  lasted  a  whole  day,  we 
should  talk  of  a  "pet  day." 

During  the  day  the  sun  is  burning  hot. 

"  When  the  sun  burns  more  than  usual,  rain  may  be 
expected." 

During  the  night  white  frost  is  formed,  owing  to 
calm  radiation. 

"  A  white  frost  never  lasts  more  than  three  days ;  a 
long  frost  is  a  black  frost." 

"Frost  suddenly  following  heavy  rain,  seldom  lasts 
long." 

As  the  day  advances,  after  a  white  frost,  the  air 
becomes  dull  from  the  influence  of  the  on-coining  depres- 
sion. Whence  the  saying — 

"  When  the  frost  gets  into  the  air,  it  will  rain." 

During  the  very  fine  weather  on  the  east  side  of  a 
wedge-shaped  area  there  is  often  great  visibility,  with  a 
cloudless  sky. 

"  The  further  the  sight,  the  nearer  the  rain." 

This  is  one  kind  of  visibility ;  there  is  another  class 
that  is  associated  with  a  hard,  overcast  sky,  as  we  shall 
explain  under  "  straight  isobars,"  and  also  visibility  in 
the  tropics,  which  depends  on  causes  which  cannot  be 


58  WEATHER. 

explained  here.  From  the  same  transparency  of  the 
atmosphere,  the  "  ashy  "  light  of  the  dark  portion  of  a 
new  moon  is  very  strong. 

"If  the  old  moon  embraces  the  new  moon,  stormy 
weather  is  foreboded."  Great  confidence  is  placed  in  this 
old  prognostic. 

"  I  saw  the  new  moon,  late  yestreen, 
With  the  old  moon  in  her  arm, 
And  I  fear,  I  fear,  my  master  dear, 
We  shall  have  a  deadly  storm."1 

At  the  extreme  north-west  edge  of  a  cyclone  there  is 
often  a  particular  kind  of  "refraction" — a  well-known 
sign  of  rain.  This  seems  to  be  due  to  the  cold  air  in  the 
rear  of  a  cyclone  being  much  below  the  temperature  of  the 
sea.  If  so  it  is  a  sign  of  rain,  for  the  reason  that  one 
cyclone  is  usually  soon  followed  by  another.  There  is 
another  kind  of  refraction  caused  by  a  cool  south-east 
wind  in  an  anticyclone  blowing  over  a  heated  sea,  which 
is  usually  a  sign  of  fine  weather.  This  is  a  good  illus- 
tration of  how  the  same  prognostic  may  portend  either 
good  or  bad  weather,  according  to  its  surroundings. 

If  the  cyclone  in  front  of  the  wedge  has  produced  a 
north-west  gale,  it  is  not  improbable  that  the  on-coming 
one  may  begin  with  a  south-west  gale.  Hence  the 
significance  of  the  well-known  nautical  saying  in  the 
Atlantic — 

"A  nor'-wester  is  not  long  in  debt  to  a  sou'-wester." 

In  the  cyclone  and  secondary  we  have  found  rain  of 
different  kinds  ;  so  in  the  anticyclone  and  wedge  we  have 
found  fine  weather  of  different  kinds.  Anticyclone  fine 
weather  is  almost  always  hazy,  and  is  settled  weather, 


WEATHER-PROGNOSTICS.  59 

because  the  anticyclone  itself  is  usually  stationary. 
Wedge  fine  weather  is  always  clear,  and  is  only  temporary 
because  the  wedge  is  never  stationary.  Hence  we  see 
that  when  we  talk  of  rain  and  fine  weather,  it  is  often 
necessary  to  say  what  kind  of  rain  and  what  kind  of  fine 
weather ;  and  we  find,  moreover,  that  a  knowledge  of  the 
kind  of  isobars  enables  us  to  define  the  kind  of  weather. 
In  many  discussions  on  climate  and  statistical  meteor- 
ology, we  find  that  terrible  confusion  is  caused  by  mixing 
up  together  all  kinds  of  good  and  bad  weather. 

The  prognostics  which  are  associated  with  a  wedge  are 
almost  less  liable  to  failure  than  those  which  accompany 
other  shapes  of  isobars.  When  they  do  fail,  it  is  usually 
from  a  sudden  break-up  of  all  the  existing  distribution  of 
pressure. 

STRAIGHT  ISOBARS. 

Straight  isobars  are  so  called  because  the  isobars  have 
no  curvature.  The  trend  of  the  lines  may  be  in  any 
direction,  and  so  may  their  slope.  For  instance,  the  lines 
may  lie  east  and  west,  but  the  slope  may  be  either  towards 
the  north  or  towards  the  south.  In  our  general  diagram 
(Fig.  1)  of  all  the  fundamental  shapes  of  isobars,  we  drew 
some  straight  isobars  sloping  to  the  south.  In  temperate 
regions  this  slope  is  uncommon,  while  a  slope  to  the  north 
or  north-west  is  very  common.  We  have  therefore  selected 
an  instance  of  a  northerly  slope  for  our  diagram  of  straight 
isobars  in  Fig.  8,  and,  as  before,  have  written  in  words  the 
kind  of  sky  and  weather  which  we  find  in  different  parts 
of  the  slope.  In  all  the  other  shapes  of  isobars  which  we 


60 


WEATHER. 


have  hitherto  described,  the  lines  enclose  an  area  of  high 
or  low  pressure,  while  in  straight  isobars  the  lines  only 
mark  the  position  of  what  may  be  called  a  barometric 
slope. 

On  turning  to  Fig.  8,  it  will  be  seen  that  while  the 
pressure  is  high  to  the  south,  it  is  generally  low  to  the 


Cloudy-  Visibility          Audibility 


falling 


Hard 


Feathery 


Cirrus 


300 


Blue  Sty 

FIG.  8. — Straight  isobar  prognostics. 

north,  without  any  definite  cyclonic  system,  and  that 
the  isobars  run  straight  nearly  east  and  west,  with  a  slope 
towards  the  north.  The  wind  is  from  the  south-west  or 
west,  and  usually  strong  and  gusty,  but  short  of  a  gale. 
On  the  high-pressure  side  the  sky  is  blue;  then  as  we 


WEATHER-PROGNOSTICS.  61 

approach  the  low-pressure,  feathery  cirrus,  or  some  form 
of  windy  sky,  makes  its  appearance,  while  a  blustery  wind 
whirls  the  dust  or  blows  the  soot  down.  The  falling  of 
soot  refers  to  blacks  falling  out-of-doors  and  coming  into 
windows  from  being  blown  about.  Sometimes,  in  very 
damp  weather,  soot  seems  to  fall  from  condensation  of 
vapour  on  itself,  and  at  other  times  masses  of  soot  fall 
down  a  chimney  from  the  action  of  hail  or  very  heavy 
rain. 

Getting  still  nearer  the  low-pressure,  the  sky  is  found 
to  be  gathering  into  hard  strato-cumulus,at  first  with  chinks 
between  its  masses,  through  which  divergent  rays  stream 
down  under  the  sun,  which  is  spoken  of  as  "  the  sun 
drawing  water."  Sometimes,  especially  in  winter,  these 
rays  are  lurid,  and  the  appearance  of  the  sky  is  then  very 
striking.  This  prognostic  is  common  all  over  Northern 
Europe,  and  in  Denmark  takes  the  form  of  "Locke 
is  drawing  water."  Loki  is  a  well-known  demi-god  in 
the  Scandinavian  Eddas,  so  that  we  have  here  a  direct 
survival  of  mythic  speech.  This  hard  strato-cumulus  is 
especially  characteristic  of  straight  isobars  in  Great 
Britain. 

At  the  same  time  there  is  often  great  "visibility," 
with  a  hard,  overcast  sky  and  moderately  dry  air,  in 
which  the  cloud  seems  to  play  the  part  of  a  sunshade, 
for  as  soon  as  the  sun  comes  out  the  clearness  of  distant 
objects  diminishes.  This  visibility  must  not  be  confounded 
with  the  visibility  already  described  with  a  cloudless  sky, 
which  occurs  with  wedge-shaped  isobars. j 

Simultaneously  we  often  find  "  audibility."  This  dis- 
tinctness of  distant  sounds  must  be  carefully  distinguished 


62  WEATHER. 

from  sounds  which  are  not  usually  heard,  being  brought 
up  by  the  wind  coming  from  a  rainy  quarter.  For  instance, 
the  whistle  of  a  railway-train  to  the  south  of  a  house  will 
not  be  usually  heard  with  the  normal  south-west  wind  of 
Great  Britain ;  but  when  the  wind  backs  to  the  south  in 
front  of  a  depression,  then  the  noise  will  be  heard ;  and 
though  this  will  be  a  good  prognostic,  still,  it  is  not  true 
audibility. 

"When  the  gradients  are  very  steep,  a  little  rain  some- 
times falls  with  straight  isobars,  generally  in  light  showers, 
with  a  hard  sky. 

Though,  as  a  matter  of  convenience,  we  have  described 
the  sequence  of  weather  as  we  proceed  from  the  high  to 
the  low  pressure,  it  must  be  clearly  understood  that  it 
does  not  represent  the  sequence  of  weather  to  a  single 
observer,  but  rather  what  the  weather  will  be  simultane- 
ously in  different  parts  of  the  country  ;  for  instance,  that 
if  there  is  cirrus  in  London,  there  may  perhaps  be  a 
lurid  sky  in  Edinburgh. 

But  now  audibility,  visibility,  whirling  dust,  and  lurid 
chinks  with  divergent  rays  are  well-known  signs  of  rain 
almost  all  over  the  world,  so  we  have  to  explain  why  the 
appearance  of  the  sky  in  straight  isobars  is  a  sign  of  rain. 
It  is  found  by  experience  that  straight  isobars  are  never 
persistent,  and  that,  practically,  the  district  which  they 
cover  one  day  will  be  traversed  by  a  cyclone  the  next 
day.  It  does  not  follow  that  the  cyclone  is  necessarily  in 
existence  when  we  observe  the  straight  isobars ;  but,  from 
the  nature  of  weather-changes,  straight  isobars  seem  to 
be  an  intermediate  form  of  atmospheric  circulation  which 
precedes  the  formation  of  a  cyclone. 


WEATHER-PROGNOSTICS.  63 

We  cannot,  therefore,  draw  a  section  across  straight 
isobars  and  say  that  it  will  give  the  sequence  of  weather 
at  any  place,  for  we  are  not  dealing  with  a  moving  form 
of  pressure,  but  with  a  transitional  state  of  things  which 
cannot  last  long.  The  chief  interest  of  these  rain  prog- 
nostics lies  in  the  contrast  which  they  present  to  those 
associated  with  a  cyclone.  While  those  in  a  cyclone  are 
accompanied  by  an  almost  ominous  calm  and  a  dirty, 
murky  sky,  these  are  associated  with  a  hard  sky  and 
blustery  wind,  of  which  it  would  be  ordinarily  remarked 
"  that  the  wind  keeps  down  the  rain,"  or,  "  that  when  the 
wind  falls,  it  will  rain."  While,  also,  the  prognostics 
which  precede  cyclone-rain  hold  good  for  the  reason  that 
they  are  seen  in  front  of  the  rainy  portion  of  such  a 
depression,  those  associated  with  straight  isobars  hold 
good  because,  though  there  is  little  rain  actually  with 
them,  the  area  which  they  cover  to-day  will  probably  be 
covered  by  a  cyclone  to-morrow — the  conditions  being 
favourable  for  the  passage  of  depressions.  Another  point 
of  contrast  lies  in  the  comparative  dryness  of  the  air  in 
straight  isobars,  as  compared  with  the  excessive  amount 
of  moisture  which  precedes  cyclones.  The  same  remarks 
apply  to  these  as  to  the  fine-weather  prognostics  associated 
with  wedge-shaped  isobars. 

All  the  prognostics  we  have  discussed  under  this 
heading  fail  when  the  straight  isobars  are  formed  during 
a  general  rearrangement  of  the  whole  distribution  of 
pressure  over  the  northern  hemisphere,  because  a  cyclone 
may  not  then  traverse  the  district  where  the  well-known 
signs  of  rain  had  been  observed. 

The  other  fundamental  forms   of  isobars — V-shaped 


64  WEATHER. 

depressions  and  cols — are  not  associated  with  any  dis- 
tinctive prognostics,  so  we  will  defer  our  consideration 
of  these  shapes  till  a  subsequent  chapter. 

GENERAL  REMARKS. 

We  are  now  in  a  position  to  take  a  general  survey  of 
the  whole  principle  of  prognostics,  and  to  answer  the 
questions  which,  we  mentioned  at  the  commencement  of 
the  chapter,  were  formerly  considered  insoluble.  What  is 
the  place  of  prognostics  in  meteorology,  and  how  has 
modern  research  developed  their  utility?  Why  do  prog- 
nostics sometimes  fail?  Why  are  not  all  prognostics 
associated  with  increasing  damp  ?  Why  is  rain  or  fine 
weather  not  always  preceded  by  the  same  prognostic  ? 

The  details  which  we  have  already  given  abundantly 
show  that  every  portion  of  every  shape  of  isobars  has  a 
characteristic  weather  and  look  of  sky,  and  that  prog- 
nostics simply  describe  these  appearances. 

Theoretically,  then,  when  the  isobars  are  well  defined, 
we  ought  to  be  able  to  write  down  the  prognostics  which 
might  be  visible  everywhere,  but  practically  we  cannot 
do  so  completely;  and  also,  theoretically,  all  that  any 
prognostic  does  is  to  enable  a  solitary  observer  to  identify 
his  position  in  any  kind  of  atmospheric  circulation.  Thus 
the  associates  of  the  front  of  a  cyclone  or  secondary  are 
signs  of  bad  weather ;  while  those  of  the  rear  of  a  cyclone, 
or  of  any  portion  of  an  anticyclone,  are  signs  of  fine 
weather.  The  word  "front"  implies  not  only  the  idea  of 
motion,  but  also  of  the  direction  of  that  motion.  But 
here  comes  in  the  reason  why  prognostics  can  never 


WEATHER-PROGNOSTICS.  65 

develop  the  science  of  forecasting  much  further  than  at 
present.  From  the  nature  of  cyclone-motion,  as  will  be 
abundantly  illustrated  in  a  future  chapter  on  Types  of 
Weather,  these  depressions  have  a  way  of  advancing  so  far 
in  a  certain  direction,  and  then  of  either  changing  their 
front  or  else  of  dying  out  altogether.  No  prognostic  can 
give  any  clue  to  the  probability  of  either  of  these 
changes.  On  the  other  hand,  a  forecaster  in  a  central 
office,  with  synoptic  charts,  can  often  tell  when  a  cyclone 
is  going  to  be  arrested  or  deflected  from  its  previous 
course,  and  this  branch  of  the  science  is  undoubtedly 
capable  of  very  great  extension. 

Then  the  question  may  very  naturally  be  asked,  How 
far  the  introduction  of  synoptic  charts  has  developed  our 
knowledge  of  prognostics  ?  So  far  as  explaining  their 
true  nature,  the  advance  has  been  very  great ;  but  so  far 
as  increasing  their  practical  utility,  the  progress  has  been 
much  less.  The  new  explanations  have  already  been 
given.  The  principal  improvement  in  reading  the  in- 
dications of  prognostics  is  rather  that  the  observer  can 
distinguish  between  the  different  kinds  of  rain  and  fine 
weather,  and  so  give  greater  precision  to  his  previsions, 
than  that  he  can  alter  the  reputed  value  of  any  weather- 
saying.  For  instance,  if  he  sees  a  halo,  with  a  falling 
barometer  and  increasing  wind,  he  knows  that  he  is  in 
for  the  whole  sequence  of  a  cyclonic  storm ;  whereas,  if  he 
sees  a  halo,  with  a  steady  barometer  and  a  few  angry 
gusts,  he  knows  that  he  need  only  expect  heavy  rain 
without  any  great  wind. 

We  have  already  mentioned  why  prognostics  some- 
times fail,  either  from  the  alteration  of  a  cyclone's  front, 

F 


WEATHER. 


the  breaking  up  of  an  an ticy clone,  and  other  similar 
reasons.  For  those  who  are  unfamiliar  with  the  nature 
of  weather-changes,  an  actual  example  will  be  more 
acceptable  than  a  general  statement.  We  will  select  an 
instance  of  the  failure  of  a  halo-prognostic.  Let  us  look 
at  the  chart  in  Fig.  9  for  February  5,  1883,  at  8  a.m. 
At  Folkestone,  near  Dover,  in  England  (marked  F  in  the 


FIGS.  9  and  10. — Illustrating  the  failure  of  the  prognostic  that  halo 
indicates  wind  or  rain. 

diagram),  a  halo  was  visible  off  and  on  from  9.30  a.m.  to 
4  pm.,  and  that  was  due  to  the  front  of  the  cyclone 
which  is  seen  lying  off  the  north-west  coasts  of  Great 
Britain.  The  whole  of  that  day  and  night,  as  well  as  the 
succeeding  day,  was  very  fine,  so  that  the  prognostic 
might  seem  to  have  failed.  Before  the  days  of  synoptic 


WEATHER-PROGNOSTICS.  67 

charts  this  is  all  that  we  could  have  said  about  the 
matter,  but  now  we  can  explain  the  reason  why. 

In  the  first  chart  (Fig.  9)  we  see  an  anticyclone 
marked  by  the  isobar  of  30'3  ins.  over  France,  and  the 
extreme  edge  of  another  over  part  of  Norway.  The  col 
between  them  covers  Denmark  and  the  North  Sea.  The 
extreme  rear  of  a  cyclone  is  found  near  Copenhagen, 
while  on  the  north-west  of  Ireland  a  new  cyclone  of 
enormous  diameter  is  approaching.  The  halo-forming 
portion  of  this  last  has  almost  reached  Dover,  while  the 
rainy  portion  is  already  causing  precipitation  over  Ireland. 
If  the  cyclone  continued  its  course,  in  due  time  the  rain- 
area  would  reach  England ;  but  suppose  the  cyclone  stood 
still,  or  some  rearrangement  of  pressure  arrested  its 
onward  progress,  what  would  happen  then  ?  Why,  the 
halo-prognostic  at  Dover  would  fail ;  that  is,  would  not 
be  followed  by  rain  and  wind,  as  is  usually  the  case.  Now, 
this  is  exactly  what  happened.  If  we  look  at  Fig.  10, 
which  gives  the  synoptic  conditions  after  an  interval  of 
ten  hours — at  6  p.m.  the  same  day — we  see  that  the  two 
small  anticyclones  have  coalesced  into  a  single  large  one, 
which  lies  over  Sweden  and  North  Germany,  while  the 
Irish  cyclone  has  disappeared,  instead  of  moving  as  usual 
to  the  north-east;  and  that  a  barometric  slope,  with 
nearly  straight  isobars,  covers  Great  Britain  and  France. 
In  consequence  of  these  changes,  the  weather  remained 
fine  all  day  near  Dover,  and  so  the  prognostic  appeared 
to  fail. 

We  will  explain,  in  another  chapter  on  Forecasting  by 
Synoptic  Charts,  why  a  forecaster  in  a  central  bureau 
could  not  have  announced  with  certainty  that  there  would 


68  WEATHER. 

have  been  no  rain  in  that  portion  of  England ;  and  also 
when  and  how  prognostics  sometimes  assist  him  in  fore- 
telling rain  which  he  would  hardly  have  expected  from 
the  mere  inspection  of  the  isobars. 

These  charts  also  give  an  idea  of  the  extreme  rapidity 
of  meteorological  changes  in  a  climate  like  that  of 
Western  Europe.  The  short  space  of  ten  hours  has  been 
sufficient  not  only  to  alter  very  materially  the  general 
distribution  of  pressure,  but  also  to  form  new  configura- 
tions of  isobars.  These  charts  also  show  that  changes  of 
weather  are  not  only  caused  by  the  passage  of  well-defined 
shapes  of  isobars,  such  as  cyclones,  in  the  manner  which 
we  have  just  described,  but  also  by  the  readjustment  of 
pressure-distribution  and  the  formation  of  new  isobaric 
shapes  over  the  area  under  observation.  This  last  con- 
ception is  of  fundamental  importance. 

Then  we  come  to  the  question  why  all  rain- prognostics 
are  not  associated  with  increasing  or  excessive  damp. 
The  answer  to  this  is  that  there  are  different  kinds  of 
rain,  such  as  the  rain  in  front  of  a  primary  cyclone,  which 
is  associated  with  great  damp ;  and  the  light  showers  of 
straight  isobars,  which  are  associated  with  a  rather  dry 
air.  Also  that  some  rain-prognostics,  such  as  those 
associated  with  the  much  too  line  weather  in  front  of  a 
wedge,  owe  their  value  to  the  fact  that  a  wedge  precedes 
a  cyclone,  though  the  air  in  itself  is  tolerably  dry. 

A  similar  train  of  argument  applies  to  the  question 
why  rain  and  fine  weather  are  not  always  preceded  by  the 
same  prognostics.  We  have  just  mentioned  two  different 
sorts  of  rain,  and,  as  regards  fine  weather,  we  need  only 
mention  that  in  a  like  manner  there  are  manv  kinds  of 


WEATHEE-PROGNOSTICS.  69 

fine  weather.  For  instance,  the  formation  of  small  blue 
patches  in  an  otherwise  overcast  sky  in  rear  of  a  cyclone 
foretells  one  kind  of  fine  weather,  while  the  radiation 
phenomena  of  an  anticyclone  also  indicate  fine  weather, 
but  of  a  totally  different  sort. 

We  have  shown  why  the  value  of  the  indications 
which  prognostics  afford  can  never  be  materially  im- 
proved, but  at  the  same  time  no  advance  in  synoptic 
meteorology  will  ever  supersede  the  use  of  prognostics. 
Our  own  researches  on  hurricanes  in  the  tropics  have 
proved  that  there,  as  in  Europe,  unusual  colouration  of 
the  sky  at  sunrise  and  sunset  apparently  often  precedes  the 
formation  of  any  notable  barometric  depression  ;  so  that 
sometimes  the  indications  of  prognostics  are  ahead  of 
those  of  any  other  system  of  forecasting.  In  isolated 
stations,  and  on  board  ship  especially,  an  observer  must 
always  rely  to  a  great  extent  on  his  own  eyes  to  gather 
information  from  the  aspect  of  the  sky  as  well  as  from 
the  readings  of  his  own  barometer.  We  shall,  in  fact, 
devote  a  whole  chapter  at  the  end  of  this  book  to  the 
consideration  of  the  problem  of  how  much  weather-fore- 
casting a  solitary  observer  can  do  for  himself.  In  the 
following  chapter  we  shall  consider  a  great  many  prog- 
nostics connected  with  clouds  which  conld  not  well  be 
associated  with  definite  shapes  of  isubars. 


70  WEATHER. 


CHAPTER   III. 

CLOUDS   AND   CLOUD-PROGNOSTICS. 

'!N  this  chapter  we  propose  to  discuss  the  nature  of  clouds 
by  first  explaining  their  origin  and  the  varying  conditions 
under  which  they  are  formed. 

This  will  lead  to  a  classification  of  their  different 
shapes  and  forms,  and  give  us  a  certain  insight  into  the 
varying  velocity  and  direction  of  the  upper  currents  of 
the  atmosphere.  But  when  we  come  to  the  more  modern 
developments  of  cloud-knowledge,  we  shall  have  to  con- 
sider the  relation  of  clouds  to  the  great  areas  of  high  and 
low  pressure,  which  we  have  already  described  as  cyclones 
and  anticyclones. 

Some  of  this  we  have  already  seen  in  our  chapter  on 
prognostics,  where  we  showed  that  different  kinds  of 
cloud  are  characteristic  of  different  portions  of  cyclones, 
etc.  But  in  this  chapter  we  will  explain  how,  from  a 
study  of  cloud-motion  in  the  upper  strata,  we  are  enabled 
to  discover  much  about  the  real  nature  of  the  circula- 
tion of  the  air  in  the  different  shapes  of  isobars.  In  the 
course  of  our  remarks,  we  shall  explain  incidentally 
both  the  meaning  and  value  of  the  older  cloud-lore,  and 


CLOUDS  AND  CLOUD-PROGNOSTICS.  71 

also  the  great  development  in  the  science  of  forecasting 
by  means  of  clouds  which  has  been  made  by  recent 
researches. 


NOMENCLATURE  OF  CLOUDS. 

Unfortunately,  in  approaching  the  subject  of  cloud- 
nomenclature,  we  come  to  one  of  the  most  unsatisfactory 
branches  of  meteorology.  Though  the  words  of  Howard's 
nomenclature  are  universally  employed,  the  same  word 
is  by  no  means  always  employed  for  the  same  kind  of 
cloud ;  and  for  this  reason,  though  the  words  we  shall 
employ  to  designate  clouds  are  those  wbi'-h  are  used  by 
many,  we  shall  be  very  careful  to  describe  exactly  the 
kind  of  cloud  we  mean  by  any  particular  name. 

For  practical  purposes  clouds  are  divided  into  four 
different  classes,  according  to  their  most  obvious  differences 
of  shape ;  but  these  classes  are  only  as  a  matter  of  con- 
venience, for  in  nature  they  all  run  into  each  other  by 
imperceptible  gradations.  The  forms  are — 

1.  Cumulus.     All  cloud  which  has  a  rot  ky  or  lumpy 
look  is  either  pure  cumulus   or  must  contain  the  word 
cumulo  in  combination  with  some  other  name. 

2.  Stratus.     All  cloud  which  lies  as  a  thin  flat  sheet 
must  either  be  pure  stratus  or  contain  the  word  strata  in 
combination. 

3.  Cirrus.     All  cloud  which  has  a  wispy,  feathery,  or 
curly  look  must  either  be  pure  cirrus  or  must  contain 
the  word  cirro  in  combination. 

4.  Nimbus.     Any  cloud  from  which  rain  is  falling  is 
nimbus  in  some  form. 


72  WEATHER. 

It  must  be  fully  understood  that  these  names  and 
their  derivatives,  which  we  shall  give  presently,  do  not 
by  any  means  exhaust  all  the  varieties  of  clouds  which 
very  experienced  observers  can  detect  and  classify.  All 
that  we  propose  to  give  here  are  the  broad  distinctions 
which  anybody  can  understand,  for  all  meteorologists 
who  have  to  deal  with  corps  of  observers  are  agreed  that 
eight  or  ten  names  are  as  many  as  can  practically  be 
employed. 

We  will  first  explain  the  simple  forms  of  these 
clouds,  and  then  the  more  complicated  combinations,  such 
as  cirro-stratus,  cirro-cumulus,  cumulo-stratus,  etc.  But 
besides  giving  a  broad  classification  to  the  leading  kinds 
of  cloud,  these  terms  also  give  a  rough  relative  scale  of 
altitude.  Thus  in  practice  stratus  and  cumulus  are  usually 
the  lowest,  the  composites  the  middle,  and  cirrus  the 
highest  layer  of  cloud ;  but  no  absolute  level  can  be 
assigned  to  each  stratum  at  any  season,  or  in  any  country. 
For  instance,  cumulus  may  be  as  low  down  as  2000  feet, 
and  cirrus  as  about  12,000  feet ;  and,  on  the  other  hand, 
cumulus  may  be  formed  up  to  at  least  25,000  feet,  and 
cirrus  probably  up  to  at  least  50,000  feet ;  but  true  cirrus 
can  never  be  formed  under  cumulus,  whatever  the  relative 
latitudes  may  be. 

These  relative  heights  also  partially  determine  the 
nomenclature.  If  a  cloud  is  very  high  up,  we  have  to  add 
the  word  cirro,  to  indicate  altitude,  to  the  word  which 
denotes  the  form  only;  while  cumulo  would  suggest  a 
lower  level.  The  first  word  before  a  compound  name 
gives  the  idea  of  relative  altitude :  thus  cirro-cumulus  is 
higher  than  cnmulo-cirrus. 


CUMULUS. 

Pure  cumulus  may  be  described  as  convex  or  conical 
heaps  increasing  upwards  from  a  flat  horizontal  base,  as 
in  Fig.  11,  a.  It  is  undoubtedly  formed  by  the  condensa- 
tion of  the  summit  of  an  ascensional  column  of  vapour- 
laden  air,  as  shown  by  the  dotted  lines.  When  this  is 
cooled,  either  by  rising  into  a  colder  stratum  than  that 
from  which  it  started,  or  by  expansion,  the  water-vapow 


t 


t 


<E£^<^ 


FIG.  11. — Cumulus  and  cirrus,     a.  Cumulus,  surface  rapid.     I.  Cirrus, 
surface  rapid,     c.  Cumulus,  upper  rapid,     d.  Cirrus,  upper  rapid. 

condenses  into  cloud,  like  the  condensed  steam  from  an 
engine. 

The  flat  base  marks  the  level  where  condensation 
temperature  is  reached,  and  the  upper  rocky  summit 
represents  the  heads  of  the  air-columns  protruding  into 
a  cold  space.  A  cumulus  is,  in  fact,  the  visible  capital  of 
an  ascensional  column  of  air. 


74  WEATHER. 

There  is  one  very  remarkable  feature  of  /*11  cumulus : 
it  is  never  seen  as  such  overhead,  but  only  ou  ^he  horizon, 
or  at  a  moderate  height  above  it. 

The  reason  is  obvious,  that  as  the  flat-based  mass 
drifts  overhead,  the  flat  under-surface  hides  the  character- 
istic rocky  top,  so  that  we  no  longer  see  the  typical 
features  of  this  kind  of  cloud. 

In  Northern  Europe,  and  the  interior  of  many  conti- 
nents, cumulus  usually  only  forms  during  the  summer 
months ;  for  the  absolute  amount  of  vapour  in  the  air 
during  the  winter  months  is  rarely  sufficient  to  develop 
a  lump  of  cloud. 

EELATION  TO  CIRRUS. 

If  the  ascensional  column  is  stationary,  we  get  a  very 
curious  appearance ;  the  top  of  the  cloud  seems  to  be, 
and  is,  in  a  state  of  commotion,  but  still  the  cloud  as  a 
whole  does  not  move  in  any  direction.  This  is  very 
puzzling  at  first,  and  is  not  uncommon  before  thunder- 
storms ;  but  we  can  readily  understand  its  origin  by 
watching  the  stationary  cloud  on  a  hilltop.  Then  we 
see  the  same  contradictory  appearance — a  cloud  in  rapid 
motion,  but  never  moving  forwards.  The  reason  is  that, 
as  each  fresh  portion  of  cloud  is  projected  upwards  and 
blown  away  by  the  wind,  it  is  immediately  evaporated, 
but  a  new  column  of  vapour  instantly  takes  its  place. 
But  suppose  that,  whether  stationary  or  moving,  the 
rising  column,  after  depositing  a  certain  amount  of  its 
vapour  at  one  level,  continues  to  rise,  it  will  at  length 
reach  a  second  level,  at  which  the  condensation-point  of 


CLOUDS  AND   CLOUD-PROGNOSTICS.  75 

the  diminished  amount  of  vapour  which  it  now  contains 
will  be  again  reached,  and  a  second  layer  of  cloud  will  be 
formed  at  a  higher  level.  Sometimes  this  will  ba  deposited 
as  another  cumulus,  but  more  frequently  the  rising  column 
has  so  much  diminished  both  in  column  and  upward 
velocity  that  the  vapour  is  condensed  in  a  thin,  hairy,  or 
curly  form,  as  in  Fig.  11,  6.  This  is  pure  cirrus,  or  curl- 
cloud,  and  may  often  be  seen  floating  above  a  cumulus. 
If  the  upper  and  lower  strata  of  air  are  moving  at  an 
equal  speed,  the  cumulus,  when  once  formed,  sails  on 
without  any  change  of  shape  from  the  action  of  the  wind, 
however  much  it  may  alter  from  any  difference  in  the 
supply  of  uprising  vapour.  But  if,  as  in  Fig.  11,  a,  we  sup- 
pose an  ascensional  column  of  air  to  start  from  near  the 
earth's  surface,  and,  when  it  has  risen  nearly  to  its  con- 
densation-level, to  meet  an  upper  current  in  the  same 
direction  as  itself,  but  moving  more  slowly,  then  we  would 
get  a  flat-based  cumulus,  headed  back,  as  it  were,  like 
the  cloud,  marked  a  in  the  figure,  while  the  whole  mass 
would  move  end  on  with  the  wind. 

In  Europe  this  is  practically  only  found  in  the  rear  of 
cyclones,  and  we  may  therefore  deduce,  from  the  shape 
of  this  cloud,  that  there  the  surface  is  quicker  than  the 
upper  current. 

Suppose,  now,  that  the  column  of  air  was  attenuated 
into  a  thread,  then  under  the  same  conditions  we  should 
get  a  feathery  cirrus  marked  b  in  the  figure,  also  moving 
end  on.  If,  now,  under  similar  conditions,  the  upper 
current  is  moving  faster  than  the  lower  one,  we  should 
get  in  the  lower  strata  a  cumulus  heading  forwards  as 
marked  c ;  and  if  at  a  higher  level  the  ascensional  column 


76  WEATHER. 

was  attenuated  into  a  thread,  then  we  should  have  a  light, 
hairy  cirrus  marked  d,  moving  long-ways  with  the  wind. 
In  this  case,  sketched  from  nature,  we  may  note  that  the 
foremost  curl  of  the  cloud  is  lumpy  like  a  small  cumulus, 
instead  of  hairy  like  the  other  tufts. 

It  is  from  simple  illustrations  of  this  sort  that  we 
seem  to  find  the  connecting  link  between  some  forms  of 
cirrus  and  cumulus.  If  cumulus  is  the  visible  capital  of 
an  ascensional  column  of  air,  cirrus  is  the  visible  form 
of  the  condensation  of  a  column  attenuated  or  wire-drawn 
into  a  thread.  Of  the  three  threads  which  have  formed 
the  cloud  dt  two  have  died  out  as  hairs,  but  the  third,  a 
little  more  intense,  had  had  enough  volume  or  energy  to 
form  a  tiny  nubecule. 

A  common  case  in  which  we  can  trace  the  gradual 
development  of  fibrous  cloud  into  cumulus  occurs  on  any 
bright  morning  with  a  blue  sky  and  heavy  dew.  When 
the  sun  gains  a  little  power,  the  first  threads  of  cloud 
formed  by  the  moisture  rising  into  the  air  are  usually 
condensed  into  cirrus;  these  quickly  get  big  and  swell, 
till  in  a  very  short  time  the  sky  is  nearly  covered  with 
true  cumulus. 

The  reason  why  the  stripes  a  and  b  are  slanting  is 
that  in  them  we  have  supposed,  as  is  often  the  case,  that 
the  cloud  continues  to  rise  after  it  has  begun  to  condense  ; 
in  c  and  d,  on  the  contrary,  the  cloud  has  ceased  to  rise, 
and  the  stripe  lies  straight. 

Ley  has  shown  that  descending  threadlets  of  icy 
particles  can  be  drawn  into  wispy  cirrus,  if  they  fall  into 
a  layer  of  air  which  moves  more  or  less  quickly  than 
themselves.  The  conditions  for  such  threadlets  would  be 


CLOUDS  AND   CLOUD-PROGNOSTICS.  77 

found  if  the  upper  outflow  from  a  cyclonic  vortex  injected 
vapour  into  the  cold  still  air  of  an  adjacent  anticyclone. 

Now  for  the  weather-portent  of  pure  cumulus.  In 
our  chapter  on  prognostics  we  have  shown  that  cumulus  is 
the  specially  characteristic  cloud  of  the  rear  of  a  cyclone, 
and  it  is  then  often  associated  with  showers.  When  this 
occurs  we  have  cumulo-nimbus ;  that  is,  rain  falling  from 
cumulus  to  distinguish  it  from  strato-nimbus,  where  rain 
falls  from  stratus  cloud.  At  the  edge  of  anticyclones 
we  often  have  a  fine-weather  light  cumulus,  which  is 
apparently  formed  by  the  simple  rising  of  the  vapour 
evaporated  from  the  ground  by  the  heat  of  the  sun,  in 
contradistinction  to  the  cyclone-cumulus,  which  we  may 
suppose  to  be  produced  by  the  ascensional  currents  which 
are  generated  by  the  whirling  motion  of  an  atmospheric 
eddy.  The  form  alone  does  not  enable  us  to  say  whether 
a  cumulus  indicates  good  or  bad  weather.  This  cloud, 
like  every  other,  must  be  judged  by  its  antecedents  and 
surroundings. 

Cumulus  is  the  almost  universal  cloud  of  the  tropics, 
and  may  indicate  either  fine  weather  or  non-isobaric  rain, 
according  to  circumstances. 

FESTOONED  CUMULUS. 

There  are  two  or  three  modifications  of  cumulus  which 
it  is  important  to  notice,  as  they  throw  much  light  on  the 
nature  of  cloud-formation.  In  looking  at  them,  as  at  all 
others,  we  must  consider  the  life-history  of  a  cloud — from 
what  it  originates,  and  to  what  it  develops.  Sometimes 
the  lower  base  of  a  cumulus  assumes  a  festooned  appear- 


78  WEATHER. 

ance,  as  in  Fig.  12, 1.  In  Orkney,  this  is  known  as  the 
"  pocky  cloud,"  and  is  there  usually  followed  by  a  severe 
gale  of  wind.  In  Lancashire,  the  festoons  are  called 
"  rain-balls,"  and  are  only  considered  a  sign  of  rain.  We 
have  frequently  seen  them  in  all  parts  of  the  Tropics ;  but 
all  festooned  forms  of  cloud  are  unknown  in  Scandinavia. 
For  a  technical  international  name,  Poey  has  suggested 
"  globo-cumulus,"  while  Ley  has  proposed  the  term 

a.  i.  a.  2. 


FIG.  12. — Festooned  cumulus. 

"  mammato-cumulus,"  and  this  latter  seems  to  be  very 
suitable. 

The  origin  of  this  form  will  be  readily  understood  by 
the  following  example.  One  summer  evening  in  London, 
towards  sunset,  the  author  saw  a  flat-based  cumulus,  like 
that  marked  a  1  in  Fig.  12,  suddenly  become  festooned 
at  the  base,  and  diminished  at  the  top,  as  marked  a  2  in 
the  diagram.  A  few  minutes  later  the  whole  cloud 
evaporated,  and  the  succeeding  night  was  fine. 

The  explanation  which  immediately  suggested  itself 
was,  that  the  ascensional  current  which  formed  the  flat- 
based  cumulus  had  suddenly  failed,  and  that  the  festoons 
were  simply  the  masses  of  vapour  falling  downwards  for 
want  of  support. 

Another  very  striking  case  is  marked  b  in  the  figure, 


CLOUDS  AND  CLOUD-PROGNOSTICS.  79 

and  was  observed  before  a  shower.  Here  a  detached 
cumulus  was  observed  to  form,  first,  festoons,  and  then 
they  in  turn  degraded  into  raggy  cloud  ;  the  whole  dis- 
appeared very  shortly,  but  were  quickly  followed  by  fresh 
rain-bearing  clouds.  The  impression  was  that  the  festoons 
were  formed  by  a  sudden  drop  of  the  cloud,  and  that  the 
rag  was  produced  when  the  drop  was  less  sudden.  The 
appearance  is,  unfortunately,  not  well  rendered  in  the 
diagram.  From  many  similar  instances,  we  are  led  to 
the  conclusion  that  the  constant  condition  necessary  to 
form  festoons  is  the  sudden  failure  of  an  upward  current 
of  air,  and  then  we  can  readily  see  why  they  should  prog- 
nosticate a  storm  in  some  cases,  and  only  rain  in  others. 

Before  many  squalls  or  showei^,  we  are  all  familiar 
with  the  short  abortive  gusts  which  so  often  precede  them. 
Now,  we  have  only  to  assume  that  the  ascensional  uptake 
in  front  of  the  main  body  of  the  shower  is  as  unsteady  as 
the  surface-wind,  and  we  have  at  once  all  the  conditions 
necessary  to  form  festoons.  All  observers  are  agreed 
that  they  are  usually  formed  at  the  edges  of  cloud-masses. 
In  the  case  of  rain  or  thunder,  they  ordinarily  appear  just 
before  or  after  the  rain ;  but  when  a  gale  follows  some 
time  afterwards,  the  festoon  must  have  been  formed  l>y 
some  local  squall  or  shower,  that  bore  some  relation  to 
the  disturbed  weather  which  produced  the  gale.  In 
Orkney  the  festoons  are  usually  seen  with  the  squalls  of 
a  north-west  wind  in  rear  of  a  cyclone  ;  the  storm  they 
prognosticate  belongs  to  another  cyclone,  which  there 
usually  follows  quickly  behind  the  first.  In  the  tropics, 
of  course,  festoons  are  always  associated  with  non-isobaric 
rain  or  thunderstorms. 


80  WEATHER. 

DEGRADED  CUMULUS. 

A  very  similar  line  of  argument  applies  to  another 
well-known  sign  of  rain — the  appearance  of  cloud  shown 
in  Fig.  13,  a,  where  a  thin  stripe  of  cloud  seems  to  cross 
a  well-formed  cumulus. 

This  is  a  foreshortened  view  of  a  cumulus  (&),  and  a 
degraded  patch  of  cloud  (o).  Sometimes,  but  most 
unfortunately,  this  cloud  is  called  cumulo-stratus,  as  by 
Howard  and  others.  We  shall  presently  see,  however, 


-as=G5^ 


FIG.  13. — Cumulus,  degraded  cumulus,  and  line  cumulus,  a.  Cumulus 
crossed  by  another  cloud,  b,  c.  The  same  from  another  side. 
d.  Line  cumulus,  or  high  cumulus,  e.  Degraded  cumulus,  lens- 
shaped. 

that  it  has  nothing  in  common  with  true  stratus,  but  is  a 
mixture  of  pure  cumulus  with  a  degraded  patch  of  cloud. 
The  origin  of  this  cloud  is  very  simple.  We  have  shown 
in  the  preceding  diagram  that  a  detached  cumulus 
(Fig.  12,  a  1)  can  become  degraded  into  a  flat,  thin  mass, 
with  festooned  base ;  but  in  certain  conditions  the  failure 
of  the  rising  current  takes  place  more  gradually,  and 
then  the  base  of  the  mass  remains  flat  instead  of 
becoming  festooned. 


CLOUDS  AND  CLOUD-PROGNOSTICS.  81 

This,  too,  is  a  sign  of  rain  for  the  same  reason  as  in 
the  former  instance.  The  existence  of  failing  or  abortive 
rising  currents  is  of  itself  a  sign  of  disturbed  weather, 
and  is  really  more  of  an  accompaniment  than  a  prognostic 
of  rain.  This  cloud  is  common  in  the  equatorial  doldrums, 
and  in  any  other  part  of  the  world  where  showers  fall 
from  cumulus.  Thus  we  see  that  festooned,  raggy,  and 
this  streaked  cumulus  are  all  associates  of  rain,  and  for 
similar  reasons.  A  very  similar  degraded  cumulus-patch 
is  very  common  duriug  the  finest  weather  in  the  trade- 
wind  districts.  The  small  isolated  patches  of  cumulus, 
which  are  so  common  there,  often  seem  to  lose  so  much  of 
their  rising  impulse  that  a  rocky  top  cannot  form,  but  at 
the  same  time  the  stoppage  is  not  so  sudden,  or  the  cloud 
so  heavy,  as  to  develop  festoons.  Then  we  get  a  cloud 
nearly  flat  below,  with  a  smooth  round  surface  above,  like 
a  plano-convex  lens,  as  in  Fig.  13,  e.  But,  as  Ley  finds 
an  almost  identical  form  as  the  embryo  of  a  cumulus 
whose  rising  force  is  very  weak,  we  must  judge  the  import 
of  this,  as  of  every  other  cloud,  by  its  surroundings. 


MINOR  VARIETIES. 

Another  form  of  cumulus  is  developed  almost  at  the 
level  of  cirrus,  in  long  thin  lines  made  up  of  little 
heads  of  condensed  vapour,  sometimes  called  thunder- 
heads.  This  is  shown  at  the  right-hand  top  corner  of 
Fig.  13,  d.  It  is  only  noticed  heie  to  guard  against  its 
being  called  cirro-cumulus.  In  practice  this  cloud  is 
almost  invariably  produced  in  front  of  thunderstorms,  and 
it  is  difficult  to  see  how  it  can  be  foimed  otherwise  than 


82  WEATHER 

by  assuming  the  air  to  rise  in  a  thin  vertical  curtain.  In 
our  chapter  on  line-thunderstorms,  we  shall  find  from  other 
reasons  that  the  air  really  does  sometimes  rise  in  long 
narrow  sheets.  This  is  the  cumulus  simplex  of  Weilbach 
and  rain-cumulus  of  Howard ;  it  has  also  been  called  high 
cumulus,  line-cumulus,  and  turretecl  cumulus. 

There  is  one  other  variety  of  cumulus,  which  need 
only  be  mentioned  here. 

Sometimes  the  top  of  the  cumulus  becomes  hairy,^as 
if  it  had  been  combed  out ;  this  cannot  be  explained,  but 
is  usually  seen  over  heavy  rain.  But  occasionally  this 
peculiar  process  on  the  top  of  a  rainy  cumulus  develops 
a  sort  of  flat  sheet  of  cloud,  apparently  touching  the 
summit,  and  the  cloud  may  conveniently  be  called 
cumulo-stratus. 

STRATUS. 

We  now  come  to  the  second  variety  of  clouds,  to 
which  the  name  of  stratus  is  applied,  because  it  always 
lies  in  a  thin  horizontal  layer,  like  a  stratum  of  rock  or 
clay.  Pure  stratus  has  no  sign  of  any  hairy  or  thread- 
like structure  except  at  the  edges,  for  a  stratum  which 
shows  much  marking  would  be  cirro-stratus,  and  has 
quite  a  different  origin.  Pure  stratus  is  essentially  a 
fine- weather  cloud,  and  is  especially  characteristic  of 
anticyclones.  One  very  beautiful  variety  is  often  seen 
during  a  fine  night,  when  the  cloud  forms  thin  broken 
flakes,  something  like  mackerel  sky,  from  which,  however 
it  is  really  quite  distinct. 

In  Howard's  original  work  on  clouds,  "stratus"  was 


CLOUDS  AND   CLOUD-PKOGNOSTICS.  83 

applied  to  ground-mist,  but  that  idea  is  now  entirely  dis- 
carded by  all  meteorologists.  What  we  call  pure  stratus 
is  the  "  strato-pallium  "  of  Weilbach,  and  the  "  stratus  "  of 
Hildebrandson.  The  origin  of  this  cloud  seems  to  be 
that  when  the  air  is  tolerably  still,  and  radiation  is  going 
on,  the  general  mass  of  the  air  gets  gradually  cooler, 
till  at  last  the  temperature  is  reached  at  which  some 
stratum  touches  the  dew-point,  and  therefore  condenses 
its  moisture  into  cloud.  Sometimes  the  cloud  is  formed 
by  rising  fog. 

This  at  once  explains  both  why  the  stratum  of  clouds 
should  be  flat  and  thin,  and  why  this  form  of  cloud 
should  be  characteristic  of  anticyclones.  We  can  also 
understand  why,  under  these  conditions,  the  sky  some- 
times becomes  overcast  almost  instantaneously.  Yery 
often  a  mass  of  fine-weather  stratus  is  uniform  in  the 
centre,  but  hairy  or  striated  at  the  edges,  and  we  get  a 
cloud  indistinguishable  by  form  alone  from  some  kinds 
of  strato-cirrus,  though  very  different  in  origin  and  sur- 
roundings. Sometimes  the  lower  surface  of  a  sheet  of 
cloud  is  festooned  for  a  short  time  like  the  flat  base  of  a 
cumulus.  The  cloud  is  probably  not  then  pure  radiation 
stratus,  but  a  smooth  form  of  strato-cumulus,  which,  by 
sudden  failure  of  the  generating  current,  begins  to  fall  in 
lumps  just  like  the  festooned  cumulus  before  described. 

CIRRUS. 

The  third  primary  form  of  cloud  is  cirrus,  a  word  taken 
from  the  Latin,  and  meaning  literally  "  a  curl  of  hair."  We 
have  already  explained  the  origin  of  pure  cirrus  and  its 


84  WEATHER. 

relation  to  pure  cumulus,  together  with  the  rudimentary 
idea  of  the  formation  of  a  stripe  of  cloud  from  a  current 
of  vapour-laden  air,  which  rises  in  currents  of  different 
velocities,  but  in  the  same  direction. 

CIRRUS-STRIPES. 

When  cirrus  rises  irregularly,  and  appears  not  to  be 
all  at  the  same  level,  we  have  seen  that  it  is  then  pure 
cirrus  ;  but  there  is  a  modified  form,  in  which  more  or  less 
of  the  sky  is  covered  with  long  thin  stripes  of  cirrus,  all 
apparently  at  the  same  level.  Technically  this  is  known 
as  cirro-filum  (literally  "  hair-thread  "),  a  name  first  sug- 
gested by  Mr.  Ley,  and  the  term  is  suitable  for  inter- 
national use ;  but  we  shall  call  them  cirrus-stripes.  As 
these  are  by  far  the  most  important  form  of  cirrus  for 
forecasting  purposes,  we  shall  devote  several  paragraphs 
to  their  consideration. 

First,  as  to  their  origin.  We  have  already  explained 
how  a  stripe  can  be  formed  which  moves  end  on  to  the 
wind  that  is  propelling  it,  but  most  frequently  we  see  the 
curious  spectacle  of  a  long  stripe  of  cloud  moving  either 
broadside  on  or  obliquely  to  its  length.  As  we  must 
suppose  that  a  stripe  always  sails  with  the  wind  in  which 
it  floats,  we  have  to  find  out  how  a  stripe  can  be  formed 
which  moves  across  its  length.  At  first  sight  this  is  one  of 
the  most  puzzling  phases  of  cloud-motion.  These  forma- 
tions of  cloud  are,  however,  exactly  analogous  to  the 
smoke  left  by  a  steamer  running  before  the  wind.  If  she 
runs  faster  than  the  wind,  her  smoke  trails  behind  ;  but  if 
the  wind  blows  faster  than  she  steams,  then  the  smoke  is 


CLOUDS  AND   CLOUD-PROGNOSTICS.  85 

blown  forwards  in  front  of  her.  But  now,  suppose  her 
to  be  heading  to  the  east  with  a  south-west  wind ;  it  is 
obvious,  from  Fig.  14,  that  her  smoke  would  lie  in  a  stripe 
bearing  somewhere  between  north-west  and  south-east, 
and  would  drift  towards  the  north-east,  that  is  nearly 
at  right  angles  to  its  length.  The  smoke  that  left  the 
funnel  when  the  steamer  was  at  A  would  have  been  blown 
to  C  by  the  time  she  had  reached  B,  while  that  at  B  would 


Course  from   W. 
FIG.  14. — Formation  of  cloud.stripes. 

be  just  leaving  the  smoke-stack ;  so  that  the  whole  line 
of  smoke  would  lie  from  B  to  C,  but  drift  from  south-west 
with  the  wind.  The  angle  the  smoke  forms  with  the 
course  of  the  ship  obviously  depends  on  the  speed  of  the 
ship  and  the  velocity  of  the  wind,  so  much  so  that  we 
have  used  measurements  of  the  angle  A  B  C  to  determine 
the  velocity  of  the  wind  at  sea. 

Now,  this  is  exactly  what  happens  in  nature.  The 
ascensional  column  of  moist  air,  which  will  eventually 
form  a  cumulus,  starts  from  near  the  earth's  surface, 
drifting  with  the  wind  which  blows  there  ;  when  it  arrives 
at  a  certain  height,  it  meets  an  upper  current  moving  in 


86  WEATHER. 

a  different  direction  to  that  on  the  surface,  and  probably 
begins  to  condense  there.  The  stripe  which  would  be 
formed  under  these  circumstances  would  behave  exactly 
like  the  smoke  of  a  steamer ;.  that  is  to  say,  it  would  lie 
obliquely  to  the  wind  which  was  driving  it. 

The  direction  of  a  stripe  is  sometimes  called  the 
direction  of  its  filature,  but  we  shall  employ  the  less 
technical  term  of  "  the  lie  of  the  stripe."  The  triangle 
ABC,  Fig.  14,  is  called  the  triangle  of  filature,  and  it 
is  evident,  from  the  nature  of  the  composition  of  velocities, 
that  the  precise  direction  of  the  lie  of  the  stripe  depends 
on  the  relation  of  the  velocities  of  the  upper  and  lower 
currents. 

If  the  stripes  were  caused  by  descending  threadlets  of 
ice  or  snow,  the  above  principles  of  stripe- formation  would 
equally  hold  good. 

LIE  AND  MOTION  OF  STRIPES. 

Before  explaining  the  nature  of  the  upper  currents  in 
cyclones  and  anticyclones,  we  must  first  explain  how  to 
find  out  both  the  lie  of  a  stripe  and  the  direction  in 
which  it  is  moving,  as  both  these  points  are  important, 
and  both  rather  difficult  to  observe.  If  the  sky  is  well 
covered  with  stripes,  we  find  that  when  we  look  at  them 
lengthways  they  appear  to  converge  towards  a  point  on 
the  horizon,  while,  if  viewed  transversely  or  in  profile, 
they  appear  arched.  The  convergence,  of  course,  is  a 
matter  of  perspective. 

One  great  peculiarity  of  stripes  is  that,  while  in  their 
simplest  form  the  threads  of  which  they  are  composed 


CLOUDS  AND  CLOUD-PROGNOSTICS.  87 

lie  in  the  direction  of  its  length,  as  marked  a,  c,  in 
Fig.  15,  sometimes  the  whole  stripe  is  made  up  of  a  series 
of  cross-bars,  and  the  stripe  is  then  said  to  be  "  striated  " 
(Fig.  15,  b  and  d).  Most  frequently  these  bars,  or  striae, 
are  at  right  angles  to  the  length  of  the  stripe,  but  they 
are  also  sometimes  oblique  to  the  lie  of  the  stripe. 

Whenever  the  cloud  is  observed,  the  points  to  be 
noted  are — (1)  the  direction  or  lie  of  the  stripe ;  (2)  the 
direction  of  the  striae ;  and  (3)  the  direction  in  which  the 


FIG.  15. — Diagram  illustrating  cloud-perspective. 

stripe  as  a  whole  is  moving.  Of  these  the  first  and  third 
are  the  most  important,  the  direction  of  the  striae  being 
only  secondary. 

Cases,  however,  occur  in  which  the  whole  sky  is 
covered  with  a  cloud  reticulated  like  a  chess-board,  and  it 
is  then  difficult  to  say  which  is  the  primary  filature  of 
the  cloud.  We  will  first  consider  how  to  determine  the 
direction  of  the  stripes  and  striae,  as  that  is  far  easier 


88  WEATHER. 

than  to  discover  their  motion.  The  method  of  doing  so 
is  based  on  the  fundamental  principle  of  perspective — 
that  a  line  drawn  from  the  observer  to  the  point  on  the 
horizon  towards  which  parallel  lines  converge,  gives  the 
lie  of  the  parallel  lines.  Thus  suppose,  as  in  Fig.  15, 
that  an  observer  looking  north  saw  stripes  a,  fc,  c,  and  d 
converging  on  the  north  point  of  the  horizon,  he  would 
conclude  that  what  he  saw  was  the  perspective  view  of 
four  stripes  lying  in  a  direction  given  by  a  line  drawn 
from  himself  northwards — that  is,  north  and  south.  On 
the  one  hand,  he  would  know  that  the  striae  of  stripe  b 
were  lying  east  and  west — that  is,  at  right  angles  with  the 
filature  ;  for  these  striae  converge  nowhere,  but  are  parallel 
to  the  horizon-line  when  looking  north.  On  the  other 
hand,  he  would  see  that  the  striae  of  stripe  d  converge 
on  the  north-east  point  of  the  horizon,  and  that,  there- 
fore, the  striae  lie  north-east  and  south-west,  while  the 
stripe  as  a  whole  points  north  and  south.  For  a  similar 
reason,  he  would  know  that  the  single  stripe  e  lay  also 
from  north-east  to  south-west. 

The  above  are  very  striking  instances  of  the  deceptive 
nature  of  perspective,  for  in  stripe  I,  where  the  striae  are 
really  at  right  angles  to  the  strips,  they  appear  oblique ; 
while  in  stripe  d,  where  they  are  really  oblique,  they 
look  as  if  they  were  nearly  at  right  angles  to  the  lie  of 
the  cirrus.  The  point  on  the  horizon  towards  which 
stripes  or  striae  converge,  is  called  their  vanishing-point, 
or,  more  shortly,  their  V-point.  Some  observers,  how- 
ever, prefer  the  term  "radiation-point,"  and  talk  of  the 
radiation  of  cirrus.  Had  the  stripes  been  viewed  looking 
straight  either  east  or  west,  they  would  have  presented 


CLOUDS  AND   CLOUD-PEOGNOSTICS.  89 

the  appearance  of  an  arch,  whose  flat  top  bore  due  east 
or  west,  and  whose  ends  pointed  north  and  south. 

A  very  simple  way  of  learning  cloud-perspective  is  to 
stand  at  the  end  of  a  long  room  and  assume  that  you  are 
looking  north  ;  then  you  see  at  once  that  the  lines  of  the 
two  cornices,  on  either  side  of  the  room,  converge  towards 
the  north  ;  and,  if  you  can  suppose  striated  lines  like  those 
of  Fig.  15  to  be  painted  on  the  ceiling  lengthways  to  the 
room,  you  will  see  that  the  striaB  would  converge  in  the 
manner  shown  in  that  diagram. 

Now  for  the  more  difficult  question  of  finding  the 
motion  of  the  stripe. 

Usually,  the  motion  of  the  stripe  is  not  in  the  direction 
of  its  length.  Frequently  it  moves  broadside  on — that  is, 
at  right  angles  to  its  length,  but  more  frequently  at  an 
oblique  angle  to  its  length. 

The  most  accurate  observations  can  be  made  when 
clouds  are  exactly  overhead.  Then,  of  course,  there  is 
no  delusive  perspective,  and  the  direction  from  which  the 
cloud  comes  is  the  direction  wanted.  A  stripe  moving 
obliquely  to  its  length  is,  however,  always  a  difficult 
subject. 

In  most  cases,  however,  it  is  impossible  to  catch  a 
cloud  just  overhead,  and  even  then  it  is  most  incon- 
venient to  observe  ;  so  that  as  a  rule  we  must  use  clouds 
at  a  moderate  elevation,  and  allow  for  perspective.  The 
perspective  of  motion  is,  of  course,  the  same  as  the  per- 
spective of  shape;  that  is,  the  point  from  which  the 
motion  appears  to  diverge  is  the  point  from  which  the 
cloud  is  really  moving,  and  the  point  to  which  the  motion 
converges  is  the  point  to  which  the  clouds  are  travelling. 


90  WEATHER. 

For  instance,  take  an  easy  case,  when  fine  detached 
cumulus  is  moving  rapidly  from  the  north.  If  we  look 
anywhere  towards  the  north-west,  a  glance  at  Fig.  15  will 
show  that  a  cloud  moving  along  one  of  the  lines  diverging 
from  the  V-point  would  appear  to  have  some  motion  from 
the  east  as  well  as  north  ;  in  fact,  would  look  like  a  north- 
east motion.  Conversely,  when  looking  towards  the 
north-east  at  a  cloud  like  d,  the  motion  would  appear  to 
be  somewhere  from  the  north-west ;  but  if  looking  due 
north,  as  at  e,  the  cloud  would  appear  to  rise  straight  out 
of  the  horizon.  The  rule  therefore  is,  watch  the  point 
from  which  the  motion  of  the  clouds  seems  to  diverge, 
and  that  is  the  direction  from  which  they  are  really 
moving.  Sometimes  it  is  more  convenient  to  determine 
the  point  towards  which  the  clouds  converge,  the  V-point 
giving  then  the  direction  towards  which  the  motion  is. 
If  possible,  however,  the  point  of  divergence  should  be 
selected  as  being  the  easier  to  observe. 

The  reason  why  the  motion  of  cirrus- stripes  is  so 
much  more  difficult  to  determine  than  the  direction  of  their 
filature  is  that,  being  very  narrow,  you  get  only  a  very  short 
line  from  which  to  estimate  the  V-point,  if  the  motion  is 
not  in  the  direction  of  the  stripe's  length.  For  instance, 
in  Fig.  15,  where  the  dotted  lines  denote  the  divergence 
motion  of  the  different  stripes,  suppose  that  we  had  been 
able  to  watch  the  motion  of  the  stripe  a  as  it  drifted  past 
a  star  or  any  fixed  point,  we  should  find  that  the  line  of 
motion  was  coincident  with  the  length.  The  motion  of 
the  stripe  is,  therefore,  from  the  V-point — that  is,  from 
the  north.  On  the  other  hand,  if  we  watched  the  stripe 
e,  which  is  really  moving  only  partially  sideways,  though 


CLOUDS  AND  CLOUD-PKOGNOSTICS.  91 

apparently  it  moves  at  right  angles  to  its  length,  we 
should  only  have  the  short  length  of  the  dotted  line 
which  passes  through  the  stripe  to  produce  by  eye  till  it 
cuts  the  horizon  from  which  to  estimate  the  V-point. 

Then  consider  the  increased  complication  when  the 
stripe  is  striated.  In  stripe  &,  though  the  motion  of  the 
stripe  coincides  with  its  length,  each  bar,  or  stria,  would 
appear  to  lie  obliquely  to  the  line  of  its  motion,  though 
really  moving  at  right  angles  to  its  length.  On  the  other 
hand,  the  striae  of  stripe  d  appear  to  move  at  right  angles 
to  their  length,  when  they  really  move  obliquely. 

Another  and  sometimes^  even  greater  difficulty  arises 
from  the  changes  which  are  going  on  in  the  cloud  itself. 
If  we  take  two  photographs  of  a  cloud  at  an  interval  of 
only  three  minutes^  it  is  sometimes  impossible  to  identify 
the  same  portion  of  the  cloud  in  the  two  pictures.  When, 
therefore,  we  get  the  still  more  complicated  case  of  an 
obliquely  striated  stripe,  which  moves  very  slowly  at  a 
considerable  angle  to  its  filature,  we  can  readily  under- 
stand that  the  true  character  of  its  motion  can  only  be 
determined  under  favourable  circumstances  and  by  a 
skilful  observer. 

Such  are  the  fundamental  principles  on  which  all 
observations  on  cloud-motion  depend.  The  observer 
must  not  be  deterred  by  difficulties  at  first  starting.  If 
he  begin  by  taking  simple  cases  of  fast-moving  clouds, 
and  then,  after  he  has  fully  realized  the  meaning  and 
importance  of  the  V-points,  tries  more  difficult  cases,  he 
will  soon  attain  such  proficiency  as  will  enable  him  to 
make  valuable  observations  in  the  most  recent  branches 
of  modern  cloud-science. 


92  WEATHER, 

If  possible,  the  velocity  of  the  upper  clouds  should  be 
noted,  for  quickly  moving  upper  clouds  are  a  sign  of 
much  worse  weather  than  slow-going  ones. 


KELATION  TO  CYCLONES  AND  ANTICYCLONES. 

It  has  been  found,  as  a  matter  of  observation,  by  Ley 
and  others,  that  the  lie  of  cirrus-stripes  bears  a  tolerably 
constant  relation  to  the  shape  of  isobars  over  the  locality 
where  they  are  seen.  It  is  from  this  circumstance  that 
cirrus  derives  its  great  forecasting  value ;  also  from  the 
fact  that  it  is  the  first  cloud  which  appears  in  a  sky 
which  had  been  previously  blue.  Hildebrandson  finds 
the  following  deviation  of  the  stripe  to  the  isobar  out  of 
171  observations.  The  stripes  whose  angles  of  deviation 
are  greater  than  45°  are,  of  course,  less  nearly  parallel 
to  the  isobar  than  those  whose  angle  is  less  than  45°. 

CYCLONES 
(minima). 

'  ANTICYCLONES      ^ * ^          WEDGE 

DEVIATION.  (maxima).     Front.          Rear.        ISOBARS.  TOTALS. 

Angle  greater  than  45°     ...       58  5  8  3  74 

Angle  less  than  45°          ..,       12  18  29  38  97 

Total          70  23  37  41          171 

Consequently,  cirrus-stripes  lie  in  regions  of  maximum 
pressure  most  often  nearly  perpendicular  to  the  isobar, 
while  round  minima  and  along  wedges  they  are  more 
nearly  parallel  to  the  isobars. 

To  explain  the  reason  of  this,  we  must  now  show  the 
relation  of  the  upper  to  the  lower  currents  in  cyclones 
and  anticyclones.  Our  knowledge  of  the  upper  currents 
has  been  deduced  entirely  from  cirrus  observations. 


CLOUDS  AND   CLOUD-PROGNOSTICS. 


93 


VERTICAL  SUCCESSION  OF  AIR-CURRENTS. 

In  Fig.  16  we  give  a  diagram  of  the  surface  and 
highest  currents  in  both  a  cyclone  and  an  anticyclone,  as 
deduced  by  Ley,  Loomis,  and  Hildebrandson  of  Upsala. 

The  solid  arrows  denote  the  surface-winds,  while  the 
highest  currents  are  given  by  the  dotted  arrows ;  and  the 
two  arrows  are  supposed  to  diverge  from  the  point  of 
observation.  In  a  few  places,  where  the  velocity  of  the 
two  currents  is  usually  different,  we  have  drawn  the 
respective  arrow  of  different  lengths,  otherwise  the  arrows 


d  , 


Cyclone.  A  n  ti  cyclone. 

FIG.  16. — Surface  and  highest  currents  over  cyclones  and  anticyclones. 

must  be  supposed  to  give  only  the  relative  directions  of 
the  winds. 

First  for  the  cyclone.  Let  us  consider  the  nature  of 
the  surface-winds,  as  shown  by  the  solid  arrows.  We  see 
at  once  that  on  the  whole  the  direction  of  the  surface- 
wind  may  be  described  as  an  ingoing  spiral,  more  incurved 


94?  WEATHER. 

in  the  right  front  than  in  any  other  portion,  and  that  in 
all  parts  the  wind  is  a  little  less  incurved  the  nearer  we 
approach  the  centre.  In  the  diagram  we  have  assumed 
that  the  cyclone  is  moving  due  west,  and  that  it  is  also 
truly  circular.  The  dotted  arrows,  on  the  contrary,  show 
that  the  wind  in  the  upper  strata  blows  in  an  irregular 
spiral  outwards ;  and  that,  while  in  front  of  the  cyclone 
the  upper  winds  are  very  much  inclined  outwards,  in 
rear  they  are  very  nearly  parallel  to  the  surface-currents. 

If  we  had  put  in  arrows  to  show  the  direction  of  the 
lower  cloud  which  floats  from  6000  to  8000  feet  above 
the  earth,  we  should  have  found  that  intermediate  layer 
moving  almost  exactly  parallel  to  the  isobars — that  is, 
nearly  in  a  circle.  We  also  know,  from  other  observations, 
that  the  upper  cirrus-current  in  front  of  a  cyclone  is  much 
nearer  the  surface  than  the  same  current  in  rear  of  the 
centre.  Now,  when  we  come  to  look  at  the  direction  of 
cirrus-stripes,  as  given  approximately  by  an  imaginary 
line  drawn  through  the  points  of  each  pair  of  arrows,  we 
see  at  once  that  in  the  outer  circle  especially,  as  at  &,  c,  d, 
the  cirrus-stripes  will  lie  at  less  than  45°  to  the  isobars,  at 
the  point  from  which  the  arrows  diverge  in  the  diagram 
if  stripes  are  formed  as  shown  in  Fig.  14. 

In  practise  the  stripes  are  more  nearly  parallel  to  the 
isobars  than  would  appear  from  the  generalized  diagrams, 
as  the  majority  of  cyclones  are  not  circular,  but  oval ;  and 
that  the  effect  of  an  intermediate  current  of  air  less 
incurved  towards  the  centre,  would  be  to  make  the  stripe 
less  at  right  angles  to  the  isobar  than  would  appear  from 
the  diagram.  Note,  also,  that  in  front  the  surface-winds 
are  slower  than  the  upper  ones,  while  in  rear  the  surface 


CLOUDS  AND  CLOUD-PROGNOSTICS.  95 

are  the  quicker ;  also  especially  that  in  every  portion 
with  one  exception,  as  at  a,  the  upper  current  is  always 
more  veered  than  the  lower  one — that  is  to  say,  that  if 
the  surface  is  east,  the  upper  will  be  more  south  of  east, 
and  if  the  surface  is  west,  the  upper  will  be  more  north  of 
west.  Or  we  may  put  it  thus :  stand  with  your  back  to 
the  wind,  and  the  upper  currents  always  come  more  from 
the  left,  and  the  higher  the  currents  the  greater  the 
amount  of  veering.  This  is  the  almost  universal  law  of 
upper  winds  in  the  northern  hemisphere  for  clouds  at  all 
levels. 

If  the  surface  is  east,  and  the  low  clouds  south,  the 
higher  cirrus  will  be  from  some  point  west  of  south  ;  but 
if,  with  the  same  surface  east  wind,  we  saw  cirrus  driving 
from  south,  we  should  know  that  the  intermediate  currents 
of  wind  cannot  be  veered  so  much,  but  must  come  from 
some  point  of  south-east.  This  sequence,  which  we  will 
call  the  law  of  vertical  succession  of  upper  currents,  is 
another  of  the  fundamental  principles  of  meteorology. 

In  certain  well-defined  cases  we  find  an  apparently 
anomalous  sequence,  but  these  need  not  be  described  in 
an  elementary  work. 

Then  for  the  anticyclone.  Referring  to  Fig.  16  again, 
we  see  that,  as  a  general  rule,  the  surface  and  highest 
winds  are  much  more  opposed  to  one  another  than  in 
cyclones,  and  therefore  in  most  cases  the  cirrus-stripes 
will  be  more  nearly  perpendicular  to  the  isobars,  as  at  e,  /, 
g.  Taking  a  general  view  of  the  surface-winds,  we  may 
say  that  on  the  whole  they  blow  spirally  outwards,  in 
the  direction  of  the  motion  of  the  hands  of  a  watch ;  that 
they  are  less  square  to  the  isobars  the  further  they  are 


96  WEATHER. 

from  the  centre,  but  that  they  are  always  more  nearly 
perpendicular  to  the  isobar  than  the  wind  in  any  portion 
of  a  cyclone. 

The  upper  currents,  on  the  contrary,  blow  spirally 
inwards,  also  in  the  direction  of  the  watch-hands,  and  also 
much  inclined  to  the  isobars. 

Now,  taking  a  general  view  of  the  relation  of  stripes  to 
isobars,  we  must  not  expect  the  lie  of  the  stripe  to  be  more 
than  a  moderately  good  guide  to  the  lie  of  the  isobars. 
Independently  of  the  fact  that  there  is  no  hard  line 
of  demarcation  between  a  cyclone  and  an  anticyclone, 
across  which  the  stripes  should  suddenly  alter  from 
parallel  to  transverse  to  the  isobars,  it  is  manifest  that 
when  so  much  depends  on  the  relative  velocities  of  the 
upper  and  lower  currents,  much  variation  in  the  lie  of  the 
stripes  must  be  expected. 

But  besides  this,  the  author  has  sometimes  found  that 
the  intermediate  current  between  the  surface  and  the 
cirrus  materially  affects  the  lie  of  the  clouds,  and  that 
the  lie  of  some  stripes  cannot  be  explained  on  this 
principle  at  all;  but,  in  spite  of  all  this,  the  above 
generalizations  on  the  lie  of  stripes  are  very  valuable. 

The  following  example,  as  observed  by  M.  Phillipe 
Weilbach,  Copenhagen,  illustrates  our  general  principles. 
Fig.  17  represents  a  portion  of  the  cirrus-stripes  as  seen 
at  1  p.m.  on  October  27,  1880,  at  Copenhagen,  foretelling 
the  bad  weather  which  occurred  on  the  following  days. 
The  sky  generally  would  be  said  to  be  covered  with 
striated  cirrus-stripes,  really  forming  a  thin  layer  of  cirro- 
stratus,  which  appeared  in  great  quantity  and  covered  the 
whole  of  the  eastern  sky  from  the  zenith  to  the  horizou, 


CLOUDS  AND  CLOUD-PROGNOSTICS.  97 

forming  long  bands,  which  diverged  from  the  noith-north- 
west  tangentially  to  the  isobars. 

The  barometer  was  at  29'6  ins.  (752  mm.) ;  the  wind 
blew  lightly  from  the  north,  while  the  relative  humidity  was 
only  sixty- seven  per  cent. ;  the  rest  of  the  sky  was  sparsely 
covered  with  fibrous  clouds  of  divers  forms.  For  several 
hours  the  bands  of  cirrus,  striated  by  fine  lines,  moved 


FIG.  17. — Converging  striated  cirrus-stripes. 

from  the  north-west ;  then  the  sky  at  Copenhagen  became 
clear  for  some  time. 

In  the  afternoon  the  preceding  appearance  was  re- 
placed by  a  del  pommele  (dappled  sky),  clearly  marked, 
but  of  a  rather  heavy  aspect,  which  also  moved  slowly 
from  the  north-west. 

At  the  same  time  the  telegraph  gave  notice  of  a 
storm  from  the  south-east  over  Ireland,  and  the  following 
day  the  tempest  raged  with  heavy  snow  over  Denmark, 

H 


98  WEATHER. 

while  the  depression  moved  to  the  south  of  that  country. 
The  landscape  in  the  figure  is  looking  up  the  Sound. 

All  this  is  very  easily  explained.  When  the  cirrus 
was  first  observed,  Denmark  was  under  the  influence  of 
the  rear  of  a  cyclone,  rather  than  of  the  wedge,  which  lay 
a  little  farther  to  the  west,  while  a  new  cyclone  was 
forming  behind  the  wedge.  The  isobars  would,  of  course, 
lie  from  north-west  to  south-east,  nearly  the  same  as  the 
cirrus.  The  episode  of  the  sky  becoming  perfectly  clear 
after  the  first  indications  of  dangerous  cirrus  is  very 
common,  and  seems  due  to  the  first  cat's-paws,  as  it  were, 
of  the  oncoming  cyclone  developing  cirrus,  and  then 
failing,  so  that  the  sky  clears  again. 

FINE  WEATHER  AND  DANGEROUS  CIRRUS. 

There  are  many  forms  of  pure,  hairy  cirrus  that 
indicate  fine  weather  all  over  the  world  ;  while  others, 
such  as  "  mare's  tails,"  "  cat's  tails,"  "  goat's  hair,"  "  sea- 
grass,"  and  •"  gashes "  (balafres),  etc.,  are  forerunners  of 
bad  weather  in  every  country. 

In  England  "  mare's  tails  "  usually  portend  wind,  and 
'goat's  hair"  only  rain;  while  "mare's  tails,"  "cat's 
tails,"  and  lalafres  precede  every  hurricane  in  the 
tropics.  "  Mare's  tails  "  are  long,  straight  fibres  of  grey 
cirrus  ;  "  cat's  tails  "  are  a  denser  bundle,  often  slightly 
striated,  so  as  to  look  like  brind lings  on  the  tail ;  "  goat's 
hair "  is  a  short  bundle  of  white  cirrus-hairs ;  while 
"  sea-grass "  and  lalafres  are  both  somewhat  similar  to 
the  above  allied  forms. 

These  all  represent  forms  of  cirrus  at  the  outskirts  of 


CLOUDS  AND  CLOUD-PROGNOSTICS.  99 

cyclones,  intermediate  between  the  pure  wisp  of  fine 
weather  and  true  cirrus-stripes,  with  the  exception  of 
"  goat's  hair,"  which  is  a  form  of  cirrifi cation  on  the  top 
of  rain-bearing  cumulus  ;  but  in  every  country  there  are 
sometimes  illusory  forms,  which  it  is  very  difficult  at 
first  to  connect  with  good  or  bad  weather.  In  England, 
on  a  fine  summer  day,  detached  cumulus,  which  has 
formed  during  the  afternoon,  will  become  very  small  and 
disappear  towards  sunset;  and  straight  fibres  of  cirrus 
will  gradually  appear  in  the  sky,  which  by  form  alone 
are  indistinguishable  from  "mare's  tails"  and  similar 
forms  of  cirrus  which  presage  wind.  All  over  the  tropics 
the  typical  sky  by  day  is  lumps  of  cumulus  floating  below 
wisps  of  cirrus,  and,  without  considering  their  surround- 
ings, the  latter  might  be  thought  to  be  indicative  of 
coming  danger.  Clouds  must  always  be  judged  by  their 
antecedents  and  surroundings.  The  gradual  growth  of 
cirrus-fibres  after  cumulus  in  England,  and  the  general 
appearance  of  the  weather  and  the  diurnal  fall  of  the 
wind,  will  usually  prevent  any  mistake  from  being  made 
between  the  "  mare's  tails  "  of  a  summer  evening  and  the 
similar  cloud  which  streaks  the  sky  on  a  windy,  gusty 
day. 

The  "cat's  tails"  which  precede  a  tropical  hurricane 
do  not  disappear  shortly  after  sunset  like  the  ordinary 
wisps,  and  when  combined  with  a  slow,  steady  fall  of  the 
barometer,  a  dangerous  storm  is  certainly  indicated. 

This  is  a  simple  case  of  what  we  find  throughout  all 
cloud-lore — that  the  same  cloud  does  not  always  indicate 
the  same  weather,  even  in  the  same  country.  Here  the 
explanation  is  doubtful.  Some  think  that  the  essential 


100  WEATHER. 

to  form  a  fibre  of  cirrus  is  a  thin  thread  of  damp  air 
rising  slowly  into  a  current  different  in  speed  or  direction 
from  that  in  which  the  air  started. 

If  the  rising  impulse  is  merely  the  effect  of  the  sun 
heating  air  near  the  ground,  the  resulting  wisp  is  a  fine- 
weather  cirrus ;  but  if,  on  the  contrary,  the  ascent  of  air 
is  due  to  the  upward  impulse  of  a  cyclone,  then  the 
bundle  of  cirrus-fibres  indicates  wind  and  rain. 

Others  consider  that  the  advent  of  damp  upper  currents 
in  front  of  a  cyclone  induce  the  condensation  of  vapour 
at  a  high  level  into  icy  particles,  which  latter  are  drawn 
into  wisps  of  cirrus  as  they  descend  into  lower  strata. 

We  believe  that  cirrus  may  be  formed  by  both 
methods,  but  it  is  impossible  to  pronounce  definitely  on 
the  subject,  till  we  know  more  of  the  mechanism  of  a 
cyclone. 

CIRRO-STRATUS. 

We  now  eome  to  the  composite  forms  of  clouds,  and 
here,  unfortunately,  we  find  the  utmost  confusion  in  the 
words  applied  by  different  meteorologists  to  the  same 
clouds.  We  will  first  begin  with  cirro-stratus.  By  this 
we  mean  a  thin  stratum  of  cloud  which,  instead  of  being 
uniform  like  pure  stratus,  is  composed  of  fibres  of  cirrus 
in  any  complexity,  but  not  of  streaked,  fretted,  or  speckled 
nubecules. 

Sometimes  the  fibres  of  cirrus  interlace,  and  give  this 
cloud  a  reticulated  appearance  like  a  woven  cloth,  and 
the  variety  of  forms  is  unlimited.  The  cloud  we  call 
cirro-stratus  is  practically  identical  with  what  Howard 


CLOUDS  AND  CLOUD-PROGNOSTICS.  101 

and  Hildebrandson  call  "cirro-stratus,"  and  almost  co- 
extensive with  the  cirro-velum  of  Ley.  As  to  the  origin 
of  cirro-stratus,  we  can  say  little  with  certainty.  As  a 
matter  of  observation,  it  is  usually  formed  in  front  of 
cyclones  or  secondaries.  When  the  sun  or  moon  shine 
through  it,  we  generally  find  that  a  halo  is  formed,  and 
then  we  may  conclude  with  certainty  that  it  is  composed 
of  frozen  particles  of  vapour.  The  difficulty  is  to  explain 
the  innumerable  forms  which  it  assumes,  and  the  rapid 
changes  which  it  undergoes.  Though  we  are  obliged  to 
employ  the  word  strata  to  describe  this  cloud,  because  it 
forms  a  thin  layer,  it  is  extremely  doubtful  whether  its 
formation  has  much  in  common  with  that  of  pure  stratus, 
which  we  have  seen  is  due  to  the  radiation  of  anticyclones. 
It  does,  however,  seem  to  have  something  in  common 
with  pure  cirrus,  and  still  more  with  cirrus-stripes ;  but 
we  cannot  say  why  the  front  of  a  cyclone  should  develop 
stratiform,  and  the  rear  cumulo-form,  clouds. 

Sometimes  cirro-stratus  is  formed  lower  down,  and 
more  compact  in  structure,  when  it  should  be  called 
strato-cirrus.  This  is  unknown  in  Scandinavia,  but  quite 
common  in  some  parts  of  the  tropics. 

ORIGIN  OF  STKIJE. 

With  regard  to  the  striae  which  we  find  both  in  cirrus- 
stripes  and  in  cirro-stratus,  the  only  reasonable  suggestion 
which  has  been  proposed  to  account  for  their  formation 
is,  that  a  stripe,  or  a  thin  stratum  of  ice-dust,  may  some- 
times be  supposed  to  be  relatively  at  rest  to  a  wind  more 
rapid  than  itself,  which  may  strike  it  suddenly.  Then 


102  WEATHER. 

we  can  conceive  that  a  smooth  layer  of  cloud  might  be 
farrowed  into  small  waves  at  right  angles  to  the  wind ;  but 
this,  of  course,  would  only  account  for  striae  square  to  the 
stripe,  and  not  for  oblique  markings.  We  often  see  an 
apparently  structureless  patch  of  cirro-stratus  suddenly 
become  striated,  as  if  a  cat's  paw  of  wind  had  blown  on  it 
like  a  gust  on  a  pond. 

But,  as  a  matter  of  fact,  striae  are  as  often  as  not 
oblique  to  the  lie  of  a  stripe,  and  to  the  direction  of 
the  motion  of  cirro-stratus.  Here  also  the  only  rational 
suggestion  is  that  the  oblique  striations  are  in  some  way 
the  effect  of  an  upper  current,  which  moves  in  a  different 
direction  to  that  on  the  surface,  and  forms  cloud  rolls. 

There  is  certainly  something  to  the  eye  about  the 
sideways  motion  of  some  cirrus-stripes  that  is  not  the 
same  as  the  drive  of  a  detached  cumulus  before  the  wind. 
If  one  is  really  propagated  by  a  dynamical  disturbance, 
while  the  other  merely  floats  in  an  air-current,  the 
difference  would  probably  be  explained.  If  this  can  ever 
be  satisfactorily  worked  out,  we  should  get  the  motion  of 
higher  currents  more  accurately  than  at  present,  for  now 
we  always  assume  the  motion  of  a  cloud  is  the  same  as 
that  of  the  wind  which  drives  it  along. 

Sometimes  a  succession  of  rising  threads  of  air,  one 
behind  the  other,  form  nearly  vertical  parallel  fibres  of 
cirrus,  which  must  not  be  mistaken  for  horizontal  striae. 
All  observers  are  agreed  that  the  fact  of  striation,  or 
reticulation,  is  of  no  practical  importance  in  forecasting 
weather  from  clouds,  so  that  we  do  not  make  definite 
varieties  of  these  forms. 


CLOUDS   AND    CLOUD-PROGNOSTICS.  103 

ClRBO-CUMULUS. 

The  next  great  class  of  compounds  is  cirro-cumulus. 
By  this  we  mean  a  broken  layer  of  cloud,  at  a  high  or 
middle  level,  of  which  the  component  masses  are  not 
fibrous  like  cirro-stratus,  but  more  or  less  rounded  or  rolled, 
though  without  any  of  the  rocky  look  of  pure  cumulus. 

For  this  reason  the  term  cirro-cumulus  is  to  a 
certain  extent  unfortunate;  but  we  are  almost  obliged 
to  use  the  word,  so  as  not  to  introduce  new  expressions, 
and,  so  long  as  it  is  conventionally  recognized  what  kind 
of  cloud  is  meant  by  cirro-cumulus,  it  does  not  so  much 
matter  if  the  word  is  not  quite  logical.  The  misfortune 
of  the  word  "  cirro-cumulus  "  is  that,  even  excluding  the 
small  high  cumulus  that  sometimes  grows  out  of  hairy 
cirrus,  and  which  we  have  described  as  linear,  or  high, 
cumulus,  there  are  still  two  rather  distinct  forms,  to 
either  of  which  the  definition  we  have  given  of  cirro- 
cumulus  applies. 

The  first  kind,  and  far  the  commoner  all  over  the 
world,  is  composed  of  rolled  masses  of  cloud,  with  a  fleecy 
appearance,  that  are  universally  known  in  different 
languages  as  '* wool-pack,"  "sheep,"  "lambs,"  or  by 
similar  terms.  This  is  the  cirro-cumulus  of  Fitzroy, 
Weilbach,  Hildebrandson,  and  of  Howard.  The  clouds 
called  nubes  hiemales  by  Weilbach,  are  a  variety  of  this 
type  that  is  formed  with  great  persistency  over  Scandinavia 
and  Northern  Europe  during  the  cold  season.  The  thin 
layer  of  cloud  is  then  at  a  moderate  altitude,  and  tends 
to  arrange  itself  in  long  parallel  bands  of  quickly  moving, 
fleec)  masses. 


104 


WEATHER. 


It  is  extremely  difficult  to  render  that  kind  of  cloud 
in  an  engraving.  Fig.  18  is,  however,  a  moderately 
successful  attempt  to  reproduce  a  photograph  of  a  fleecy 
sky.  There,  as  always,  the  cloud  has  a  more  or  less 
pronounced  tendency  to  arrange  itself  aloDg  two  lines — 


FIG.  18. — Fleecy  cirro-cumulus. 

one  for  the  length  of  the  bands ;  the  other  for  the  lie  of 
the  striae.  Sometimes  the  effect  of  these  two  crossing 
lines  is  to  give  the  individual  nubicules  which  compose 
the  whole  a  square  or  lozenge  shape,  and  the  whole  sky 
the  appearance  of  a  gigantic  chess-board. 


CLOUDS  AND  CLOUD-PROGNOSTICS.  105 

We  can  say  little  with  certainty  as  to  the  formation 
of  this  fleecy  sky,  though,  in  a  general  way,  there  seems 
to  be  little  doubt  that  both  the  woolly  look  and  the 
striation  are  due  to  the  contact  and  rolling  friction  of  two 
layers  of  air  moving  in  different  directions.  Fleecy 
clouds,  though  apparently  so  different  in  form,  are  really 
not  very  far  removed  from  wispy  cirro-stratus.  We  often 
see  in  England  wispy  clouds  develop  rapidly  into  fleecy 
ones  for  a  few  minutes,  and  then  back  again  into  wisps 
and  curls;  but,  as  a  rule,  cirro-stratus  develops  into 
strato-cumulus,  and  is  practically  a  sign  of  worse  weather 
than  fleecy  cirro-cumulus. 

We  know  by  observation  that  fleecy  cirro-cumulus  is 
chiefly  formed  in  the  temperate  zone  on  the  edges  of 
anticyclones,  and  also  before  thunderstorms  and  some 
forms  of  non-isobaric  rain.  These  are  both  cases  in  which 
there  would  be  upper  currents  varying  much  in  direction 
from  the  surf  ace- winds,  while  the  rapidity  of  motion 
would  depend  upon  circumstances.  This  enables  us  to 
explain  the  following  set  of  widely  reputed  prognostics. 

"  If  woolly  fleeces  spread  the  heavenly  way, 
Be  sure  no  rain  disturbs  the  summer's  day." 

Or  the  provincial  French  saying,  "El  ciel  pecoun  pro- 
mete  un   bel  matin."     But,  on  the  other   hand,  Virgi 
("Georg."  i.  397)  considers  it  a  sign  of  rain  if  it  should 
happen  that — 

"  Tenuia  .  .  .  lanse  per  cselum  vellera  ferri." 

And  so  in  the  neighbourhood  of  Pisa  they  say,  "Cielo  a 
pecorelle,  Acqua  a  catinelle ; "  and  in  the  Tyrol,  "  Sind 
Morgens  Himmelschaflein,  wird's  Nachinittags  hageln 


106  WEATHER. 

oder  schnei'n  ; "  and  in  France  they  have  a  proverb  con- 
trary to  the  one  we  have  first  quoted : 

"  Temps  pommele,  fille  fardee, 
]STe  sont  pas  de  loiigue  duree." 

The  term  "  dappled  sky "  (del  pommele)  is  a  little 
equivocal,  and  might  refer  to  the  other  form  of  cirro- 
cumulus,  known  in  Northern  Europe  as  "  mackerel  sky." 

Anyhow,  we  have  to  reconcile  an  apparently  contra- 
dictory set  of  prognostics.  The  reason  appears  to  be 
that  in  Northern  Europe  rain  is  chiefly  cyclonic,  and 
therefore  rarely  preceded  by  fleecy  cirro-cumulus,  so  that 
the  appearance  of  that  cloud  denotes  the  edge  of  an 
anticyclone,  and  fine  weather  for  a  day  at  least.  In 
Central  and  Southern  Europe,  on  the  contrary,  fleecy 
clouds  are  usually  formed  in  front  of  secondaries,  thunder- 
storms, and  non-isobaric  rains,  so  that  their  cirro-cumulus 
is  a  sign  of  approaching  rain.  We  can  readily  imagine 
that,  both  at  the  edges  of  anticyclones  and  in  front  of 
secondaries,  thunderstorms,  etc.,  we  have  upper  currents 
moving  in  very  different  directions  to  those  on  the  surface, 
with  a  layer  of  cloud  between  them,  though  the  origin  of 
the  condensed  vapour  is  not  the  same.  In  anticyclones 
the  vapour  probably  rises  from  evaporation,  till  it  reaches 
an  altitude  where  the  temperature  falls  to  the  dew-point ; 
in  secondary  cyclones,  etc.,  the  upward  impulse  is  due  to 
the  dynamical  properties  of  cyclonic  or  other  motion. 

This  is  exactly  analogous  to  the  difference  between 
the  wispy  cirrus  formed  of  an  evening  at  the  edges  of 
anticyclones  in  fine  weather,  and  the  same  cloud  which 
precedes  a  dangerous  storm. 

In  practice  the  surroundings  are  so  different  that  the 


CLOUDS  AND   CLOUD-PROGNOSTICS.  107 

apparent  similarity  of  names  rarely  misleads  the  rno4 
ordinary  observer. 

The  second  chief  variety  of  cirro-cumulus  is  composed  of 
rounded  and  isolated  nubicules  without  any  fleecy  texture. 

This  is  the  well-known  "  mackerel  sky  "  of  Northern 
Europe;  and  when  the  cloudlets  are  a  little  angular, 
we  get  a  form  called  "  mackerel-scales."  We  may 
call  this  hard  cirro-cumulus,  to  distinguish  it  from  the 
fleecy  form  of  the  same  generic  name.  While  fleecy 
cloud  is  one  of  the  commonest,  mackerel  is  one  of  the 
rarest  skies,  so  that  we  have  not  got  a  sufficient  number 
of  observations  to  correlate  these  isolated  cloudlets  with 
any  particular  form  of  isobars  or  kind  of  rain. 

However,  all  weather-lore  connects  mackerel  with  fine 
weather,  for  even  in  rainy  Ireland  we  find  the  saying, 
"  Mackerel  sky,  twelve  hours  dry."  Why  this  should  be 
the  case  we  are  unable  to  say,  but  there  is  no  doubt  about 
the  fact. 

In  a  still  rarer  form  of  cirro-cumulus,  the  lower 
surface  of  the  general  cloud-stratum  exhibits  very  small 
pendulous  protuberances,  resembling  sacks  or  bags,  by 
which  a  part  or  even  the  whole  sky  is  festooned.  Ley 
calls  this  eirro-velum  mammatum,  but  we  may  call  it 
festooned  cirro-cumulus.  When,  near  sunset  in  the 
tropics,  these  festoons  take  up  a  rosy  tint,  and  hang  like 
pink  grapes  in  a  serene  sky,  these  clouds  can  scarcely 
be  surpassed  for  beauty. 

Sometimes  a  more  compact  form  of  fleecy  cirro- 
cumulus  is  found  at  a  lower  level,  when  the  cloud  may 
be  more  appropriately  reported  as  cumulo-cirrup,  so  as 
to  indicate  its  lower  level.  This  apparent  multiplication 


108  WEATHER. 

of  cloud-names  is  forced  on  us  by  the  necessity  of  giving 
some  idea  of  height  in  reports  of  the  motion  of  the  upper 
currents.  For  instance,  on  the  west  edge  of  an  anticyclone 
low  cumulo-cirrus  might  be  moving  from  south,  whilst 
the  higher  cirro-cumulus  would  come  from  the  south-west ; 
so  that  observations  which  reported  cirro-cumulus  and 
cumulo-cirrus  indiscriminately  would  lead  to  a  discordant 
or  erroneous  view  of  the  general  circulation  of  the  air  in 
an  anticyclone. 


STRATO-CUMULUS. 

Another  of  the  great  series  of  compounds  is  strato- 
cumulus.  By  this  we  mean  a  large  mass  of  cloud,  forming 
a  layer,  which  is  not  sufficiently  uniform  to  be  called 
stratus,  and  not  sufficiently  rocky  to  be  called  cumulus. 
This  is  the  cumulo-stratus  of  Fitzroy.  Howard's  cumulo- 
stratus  is  not  a  true  variety  of  cloud  at  all,  but  a 
compound  of  a  thin  patch  of  cirro-stratus,  resting  either 
on  the  top  of  a  cumulus  or  crossing  an  isolated  lump  of 
cumulus,  as  in  Fig.  13,  a.  The  origin  of  the  name  is 
obvious.  The  general  mass  of  the  cloud  is  a  layer,  and 
therefore  the  name  must  contain  the  word  strata,  while 
the  components  are  lumpy,  and  it  must  therefore  con- 
tain the  word  cumulo. 

This  form  of  cloud  is  typical  of  a  cyclone-front  in 
Great  Britain.  We  can  trace  its  gradual  development  in 
all  stages.  Cirrus-stripes  first  get  thicker  and  lower,  so 
as  to  form  cirro-stratus.  As  we  get  nearer  the  rainy 
portion  of  the  cyclone,  the  cirro-stratus  loses  its  fibrous 
texture,  becomes  still  denser  and  nearer  the  earth's 


CLOUDS   AND   CLOUD-PROGNOSTICS.  109 

surface,  till  at  last  all  trace  of  structure  is  lost  in  the 
irregular,  shapeless  masses  of  cloud  which  cover  the 
whole  sky.  Still  later,  the  cloud  gets  even  lower  and 
blacker,  till  rain  ultimately  begins  to  fall.  Then  the 
cloud  would  be  called  nimbus,  because  it  forms  a  layer 
and  precipitates  rain.  Sometimes,  when  the  sky  breaks 
ior  a  moment,  we  get  a  glimpse  at  the  composition  of 
this  cloud ;  we  then  see  that  it  differs  much  from  pure 
rocky  cumulus,  by  reason  of  its  flatness  and  comparative 
thinness.  We  must,  in  fact,  look  at  strato-cumulus  as  a 
development  of  cirro-stratus,  and  not  as  an  ally  or  hybrid 
of  cumulus,  though  we  have  to  use  the  word  "  cumulus  " 
in  composition. 

The  point  which  we  cannot  altogether  explain  is,  why 
in  front  of  the  cyclone's  trough  the  clouds  should  have 
such  a  marked  tendency  to  form  stratus,  while  in  rear 
the  rising  currents  take  the  form  of  well-defined  columns, 
and  produce  rocky  cumulus.  This  points  to  some 
difference  of  symmetry  between  these  two  portions  of  a 
cyclone,  and  the  only  suggestion  which  we  can  make  is, 
that  perhaps  it  may  be  partly  due  to  the  upper  currents 
in  front  of  the  trough  being  much  more  opposed  to  those 
on  the  surface  than  those  in  rear  of  the  centre,  which  are 
nearly  parallel  to  the  lower  winds;  and  partly  to  the 
forward  motion  of  the  cyclone,  as  a  whole,  meeting  the 
incurving  winds  in  front,  and  running  away  from  them  in 
rear  of  the  disturbance. 

Another  form  of  strato-cumulns  is  very  common  in 
the  tropics.  The  component  masses  of  cloud  are  more 
isolated  than  in  Great  Britain,  and  so  thin  that  when 
seen  in  perspective  each  only  looks  like  a  dark  thin  bar, 


110 


WEATHER. 


and,  with  the  brighter  intervening  spaces,  the  whole  sky 
near  the  horizon  is  striped  like  a  Venetian  blind. 

Nearer  overhead  we  see  only  the  irregular  flat  base  of 
scattered  clouds,  without  any  trace  of  arrangement  or  of 
bars.  The  difference  between  these  apparent  long  bars  and 
real  stripes  of  cirrus  can  be  detected  in  a  moment  by 
turning  in  any  direction.  The  bars  of  strato-cumulus 
follow  you  by  remaining  parallel  to  the  horizon  which- 
ever way  you  look,  for  the  linear  arrangement  is  only  an 
effect  of  perspective ;  while  cirrus-stripes  always  converge 
to  the  same  point  on  the  horizon.  Fig.  19,  which  is  a 


FIG.  19. — Strato-cumulus  j  roll  cumulus. 

fair  specimen  of  this  kind  of  cloud,  is  engraved  from  a 
photograph  by  the  author  in  lat.  18°  S.,  long.  4°  E. ;  that 
is,  in  the  south-east  trade  between  Goree  and  Cape  Town. 
We  see  at  once  that  the  sky  is  too  irregular  for  pure  stratus, 
but  that  the  masses  into  which  the  cloud  is  gathered 


CLOUDS  AND   CLOUD-PROGNOSTICS.  Ill 

have  nothing  in  common  with  pure  cumulus ;  and  also 
very  clearly  that  the  linear  arrangement  increases  towards 
the  horizon.  This  is  the  cloud  to  which  the  term  "  roll- 
cumulus  "  has  been  unfortunately  applied  in  England. 

Though  true  strato-cumulus  is  not  really  allied  to 
cumulus  at  all,  we  sometimes  see  a  cloud  of  this  type 
with  a  distinct  but  irregular  cumulus  form  in  places. 
This  must  also  be  called  strato-cumulus,  as  it  merges  by 
indistinguishable  gradations  into  the  purer  form  of  the 
same  name. 

Sometimes  also  we  get  strato-cumulus  from  a  develop- 
ment of  cumulo-cirrus  with  fine  weather  in  the  temperate 
zone ;  so  that  the  name  and  form  alone  of  this  cloud  tell 
us  little  either  of  its  origin  or  portent. 

NIMBUS. 

The  term  nimbus  need  not  detain  us  long,  and  then 
principally  to  explain  the  unfortunate  confusion  which 
has  arisen  from  the  uncertain  use  of  this  word. 

Every  kind  of  cloud  from  which  rain  falls  is  a  nimbus, 
and  there  are  practically  two  sorts — cumulo-nimbus,  the 
rocky  cumulus-cloud  from  which  rain  falls  in  squalls  or 
showers ;  and  pure  nimbus,  a  flatter  cloud,  more  like 
heavy  strato-cumulus,  that  forms  from  or  under  cirro- 
stratus  in  front  of  extra  tropical  cyclones.  Howard  calls 
nimbus  "  a  cloud,  or  system  of  clouds,  from  which  rain 
is  falling.  It  is  a  horizontal  sheet,  above  which  the 
cirrus  spreads,  while  the  cumulus  enters  it  laterally  and 
from  beneath." 

Hildebrandson  uses  the  word  in  a  more  contracted 


112  WEATHER. 

signification,  and  reserves  the  name  of  nimbus  for  the 
lower  layers  of  dark  torn  clouds  from  which  rain  falls. 
Poey  calls  the  same  broken  clouds  fracto-cumulus. 

Weilbach  designates  by  nimbus  the  property  which  a 
cloud  manifests  to  be  or  to  become  a  source  of  rain  in 
particular  circumstances,  and  then  gives  three  varieties — 
nimbo-pallium,  the  rain-cloud  in  front  of  cyclones,  which 
we  have  called  pure  nimbus  ;  nubeculse,  or  scud ;  and 
nimbo-stratus,  the  rain-cloud,  in  rear  of  cyclones,  which 
we  have  designated  cumulo-nimbus.  He  also  gives  a 
plate  marked  cumulo-nimbus,  which  is  identical  with 
our  application  of  the  same  name. 

The  reason  for  making  nimbus  a  class  of  its  own 
comes  from  the  fact  that  a  sudden  striking  change  comes 
over  the  look  of  the  upper  surface  of  a  cloud  the  moment 
rain  begins  to  fall,  the  precise  nature  of  which  we  cannot 
at  present  explain. 

The  following  remarkable  description  of  the  changes 
which  often  take  place  in  the  appearance  of  the  summit 
of  a  cumulus  when  it  commences  to  discharge  rain,  is 
given  by  Mr.  Ley : — 

"  Under  a  summer  sky  a  massive  cumulus  begins  to 
form  a  few  miles  distant  from  the  observer.  The  atmo- 
sphere being  nearly  calm  up  to  the  height  of  twelve  or 
fourteen  thousand  feet,  the  cumulus  preserves  its  hemi- 
spherical form,  and  an  enormous  aggregate  of  cloud- 
inatter  is  produced,  the  contents  of  which  may  occupy 
a  space  of  upwards  of  a  hundred  cubic  miles,  while  the 
extreme  opacity  of  the  cloud  shows  that  the  water- 
spherules  which  compose  it  are  somewhat  closely  packed. 
No  rain  falls  from  such  a  cloud  while  it  preserves  the 


CLOUDS  AND  CLOUD-PROGNOSTICS.  113 

hard  outline  of  its  upper  portions  and  its  general  hemi- 
spherical figure.  Suddenly  the  summit  of  this  cloud 
becomes  soft-looking,  and  spreads  out  laterally  in  cirri- 
form  fibres,  this  change  being  always  simultaneous  with 
the  fall  of  a  sheet  of  rain  out  of  the  cloud. 

"  The  electrical  charge  which  prevented  the  collision 
of  the  particles  composing  the  cloud  while  these  particles 
remained  spherical,  has  been  suddenly  diminished  in  the 
upper  portions  of  the  cloud  as  soon  as  these  particles  are 
congealed  into  ice-needles,  from  the  edges  and  extremities 
of  which  the  electricity  immediately  escapes. 

"  The  particles,  now  only  moderately  electrified,  unite,, 
and  in  their  rapid  descent  absorb  the  smaller  spherules 
with  which  they  come  in  contact. 

"  The  falling  rain,  and  perhaps  still  more  rapidly  the 
disruptive  discharges,  if  such  occur,  further  tend  to  '  tap ' 
the  electricity  of  the  cloud — i.e.  to  lower  the  potential  of 
the  cloud-mass — and  the  shower-making  process  continues 
till  all,  or  nearly  all,  of  the  lower  portion  of  the  cloud  has 
disappeared.  Now,  in  those  instances  in  which  there  is 
not  only  very  little  motion  in  the  lower  layers  of  the 
atmosphere,  but  also  in  the  higher,  the  ice-cloud  left  in 
the  upper  regions  of  the  atmosphere  is  a  true  cirrus,  the 
curls  and  twisted  forms  of  which  are  probably  due  to 
slight  lateral  inequalities  of  pressure  produced  by  the- 
processes  of  condensation  and  congelation.  A  cirrus  so 
produced  may  hang  nearly  motionless  for  upwards  of 
twenty-four  hours  in  the  sky,  or  may  more  commonly 
drift  very  slowly  over  districts  from  which  the  shower 
which  produced  it  was  invisible." 

Mr.  Ley  further  says  that  he  can  always  tell  by  the 


114  WEATHER. 

appearance  of  the  top  of  a  cloud  whether  it  is  discharging 
rain  or  not ;  but  the  cirrification  of  rain-cumulus  is  cer- 
tainly not  necessary  to  precipitation. 

The  allusion  to  the  discharge  of  electricity  at  the 
moment  of  precipitation  refers  to  an  idea  which  has 
much  to  support  it — that  free  statical  electricity  tends  to 
keep  small  condensed  globules  of  vapour  apart. 

Lord  Eayleigh  has  proved,  experimentally,  that 
moderately  electrified  water-drops  tend  to  coalesce,  but 
that  strongly  electrified  drops  repel  one  another.  The 
precise  bearing  of  this  on  the  formation  of  rain  cannot  be 
given,  but  it  shows  unmistakably  that  there  is  a  real  con- 
nection between  rain  and  electrical  manifestations.  We 
may,  however,  remark  that  it  is  almost  certain  that  the 
presence  of  electricity  is  quite  secondary  to  the  other 
influences  which  develop  rain. 

Electricity  may  determine  the  precipitation  of  a  cloud, 
but  it  cannot  give  rise  to  the  ascensional  current,  which 
is  the  primary  cause.  Connected  with  the  appearance  of 
the  top  of  cumuli  there  is  a  well-known  saying,  that 
"  When  clouds  look  woolly,  snow  may  be  expected." 
This  refers  to  the  tops  of  cumuli,  and  not  to  the  ordinary 
woolly  cirro-cumulus  which  is  so  so  often  seen  in  summer. 
There  is  reason  to  believe  that  this  woolly  look  is  really 
due  to  the  cloud  being  composed  of  frozen,  and  not  of 
liquid,  globules  of  water.  The  author  has  made  some 
observations  on  the  sudden  splash  of  rain  or  hail,  which 
often  comes  directly  after  a  flash  of  lightning,  and  the 
thunder-clap  which  accompanies  them.  By  measuring 
the  time  which  elapses  between  the  flash  of  lightning  and 
both  the  thunderclap  and  the  splash  of  rain  that  follows 


CLOUDS  AND  CLOUD-PKOGNOSTICS.  115 

to  one-fifth  of  a  second,  he  has  found  that  the  flash,  the 
clap,  and  the  splash  of  rain  may  be  supposed  really  to 
occur  simultaneously,  but  that  the  three  impressions 
reach  the  earth's  surface  at  different  times,  because  light, 
sound,  and  a  falling  body  all  travel  at  various  rates.  Thus 
light  travels  practically  instantaneously;  sound  at  the 
rate  of  about  1100  feet  a  second ;  while  rain-drops  fall  a 
definite  distance  in  any  given  time,  under  the  influence 
of  gravitation.  This  would  be  proved  if  we  found  that 
the  distance  of  the  origin  of  lightning,  as  measured  by 
the  velocity  of  the  sound  of  the  thunder,  was  the  same  as 
that  measured  by  the  velocity  of  falling  rain.  For 
instance,  on  one  occasion  the  interval  between  the 
lightning  and  the  thunder  was  five  seconds,  while  the  rain 
did  not  arrive  for  nineteen  seconds.  Now,  calculating 
the  distance  of  the  origin  of  the  lightning  from  the 
velocity  of  sound,  we  find  the  altitude  to  be  5500  feet ; 
while  the  distance  through  which  a  drop  would  fall  in 
nineteen  seconds  would  have  been  5800  feet.  The 
difference  is  only  300  feet,  which  is  very  little  considering 
the  nature  of  the  observations,  and  the  unknown  retarda- 
tion of  a  falling  drop  from  the  resistance  of  the  air.  In 
practice  the  thunder  always  arrives  before  the  rain;  in 
fact,  we  may  consider  that  the  same  disruptive  discharge 
of  electricity  sends  three  messages  to  the  earth  at  different 
rates,  and  to  different  senses — the  light  to  the  eye,  the 
sound  to  the  ear,  and  the  rain  to  the  touch 


116  WEATHER. 


UNCLASSIFIED  CLOUDS. 

So  far  for  the  great  subdivisions  of  cloud-forms,  but 
we  must  now  mention  a  few  minor  forms,  because  they 
have  some  importance  in  judging  weather. 

ClRRO-NEBULA,  OR  ClRRUS-HAZE. 

Sometimes,  as  a  cyclone  approaches,  in  any  part  of 
the  world,  and  we  are  very  nearly  on  the  line  of  its  path, 
we  see  a  blue  sky  first  get  white,  then  grey,  and  then 
work  up  to  drizzling  rain,  without  the  formation  of  any 
true  cloud-form.  When  this  happens,  the  sky  is  said 
popularly  to  sicken,  and  this  is  an  almost  infallible  sign  of 
rain,  and  probably  of  wind.  Mr.  Ley  has  proposed  the 
name  "  cirro-nebula,"  or  "  cirrus-haze,"  for  this  appear- 
ance, and  the  term  seems  most  appropriate.  We  may, 
however,  observe  here  the  necessity  for  our  caution  about 
the  words  cirro,  cumulo,  etc.,  conveying  a  rough  idea  of  the 
height  of  clouds. 

This  cloud  has  no  fibrous  or  hairy  structure  to  which 
the  name  of  cirro  could  be  strictly  applied ;  but  if  we  also 
lay  down  that  the  word  cirro  is  to  convey  the  idea  of  a 
high  level  cloud,  then  the  word  cirro-nebula  is  quite 
correct.  It  is  invariably  formed  at  a  great  height,  and  as 
it  nearly  always  shows  a  halo  when  the  sun  or  moon  shines 
through  it,  we  may  assume  that  it  is  composed  of  frozen 
particles,  or  ice-dust.  After  it  has  formed,  we  can  often 
watch  a  layer  of  cirro-stratus  being  formed  underneath 
the  haze.  From  all  this  we  may  draw  the  important 


CLOUDS  AND  CLOUD-PKOGNOSTICS.  117 

inference  that,  though  the  front  of  a  cyclone  is  charac- 
terized by  excessive  warmth  on  the  surface,  the  upper 
strata  are  then  very  cold. 


SCUD,  WEACK. 

Under  any  mass  of  cloud  which  is  verging  on  the 
precipitation  of  rain,  we  have  just  mentioned  that  small 
detached  clouds  are  frequently  seen  in  rapid  motion.  In 
England  they  are  called  "  scud  ;  "  in  France,  fuyards,  or 
diabletons ;  while  Poey  suggests  the  name  of  fracto- 
cumulus.  If,  instead  of  being  shapeless,  they  are  raggy, 
they  are  then  known  in  England  as  "  wrack,"  from  their 
drawn-out  appearance.  In  all  cases  it  is  obvious,  from 
the  above  description,  that  they  are  rather  associates  of 
heavy  rain-clouds  than  true  prognostics.  What  we  have 
to  explain  is  their  origin.  This  seems  to  be  simply,  that 
in  very  disturbed  weather  small  masses  of  cloud  form  like 
ordinary  ragged  clouds,  from  the  irregular  nature  of  the 
rising  currents,  while  the  apparently  very  rapid  motion 
<eomes  from  their  being  nearer  the  surface  than  ordinary 
clouds. 

CLOUD- WBEATHS. 

Sometimes,  in  front  of  certain  kinds  of  squalls  and 
thunderstorms,  we  see  a  long,  narrow  roll  of  black  cloud 
moving  rapidly,  broadside  on,  and  a  very  well-developed 
example  will  be  found  in  Fig.  56,  under  the  heading  of 
"Pamperos."  Dark  cloud-wreaths  in  a  very  much  less 
pronounced  form  are  very  common  in  England  before 


118  WEATHEE. 

certain  classes  of  squalls  and  showers,  and  in  front  of  a 
curious  light  grey  vault  of  rain-producing  cloud,  as  illus- 
trated in  Fig.  54,  in  our  chapter  on  Squalls. 

There  are,  of  course,  many  other  minute  differences  of 
the  various  classes  of  cloud,  to  which  it  is  impossible  even 
to  allude  in  an  elementary  work  like  the  present.  Our 
object  will  have  been  attained  if  we  have  succeeded  in 
explaining  the  general  principles  of  cloud-formation,  and 
the  method  of  making  prognostications  from  their  vary- 
ing appearance.  A  few  hours  spent  in  watching  the 
changing  and  degrading  forms  of  a  sky  which  is  covered 
by  detached  cumulus,  or  the  very  different  modifications 
almost  from  minute  to  minute  of  cirro-stratus,  will  better 
assist  any  one  to  understand  the  nature  of  cloud-forms 
than  reading  pages  of  the  best  printed  matter. 

We  may  conveniently  summarize  here  the  various 
varieties  of  cloud-forms  which  we  have  already  described. 
The  general  idea  of  our  classification  has  been  that, 
though  for  large  bodies  of  observers  all  practical  men  are 
agreed  that  eight  or  ten  principal  varieties  are  all  that 
can  safely  be  used,  still  more  advanced  cloud-observers 
will  not  be  satisfied  with  so  coarse  a  subdivision,  and 
that  therefore  more  minute  varieties  are  necessary. 

The  ten  principal  varieties  are  therefore  printed  in 
capitals,  while  the  minor  varieties  are  denoted  by  smaller 
letters. 

But  there  is  another  point  in  our  subdivision  of 
varieties.  Almost  all  the  smaller  varieties  are  so  rare 
or  transient  that  for  practical  purposes  they  may  be  neg- 
lected; but  if,  on  the  contrary,  the  ten  main  words  are 
restricted  to  the  forms  of  clouds  we  have  described  under 


CLOUDS  AND  CLOUD-PROGNOSTICS.  119 

them — that  is,  cumulus,  pure  rocky  cloud;  stratus,  pure 
sheet  cloud  ;  cirrus,  pure  wispy  cloud  ;  cirro-stratus,  thin, 
high,  wispy,  or  striated  sheet  cloud  of  all  sorts ;  strato- 
cirrus,  a  similar  low  cloud ;  cirro-cumulus,  fleecy  cloud  at 
high  level ;  cumulo-cirrus,  the  same,  lower  down ;  strata- 
cumulus,  extended  lumpy  cloud ;  nimbus,  low  rain-cloud ; 
cumulo-nimbus,  rocky  rain-cloud — then  the  author  can 
say,  from  an  experience  of  cloud-observation  in  all  longi- 
tudes, and  in  latitudes  ranging  from  72°  north  to  55° 
south,  that  ninety  per  cent,  of  skies  in  every  part  of  the 
world  can  be  sufficiently  accurately  defined  by  these  ten 
words. 

VARIETIES  OF  CLOUDS. 

With  the  mean  height  of  the  principal  varieties  at  Upsala 
in  summer: — 

FEET. 

/CIRRUS        ...        27,000 

Cirrus  stripes. 

HIGH     /          Cirrus  haze. 

)  CIRRO-STRATUS       27,600 

\ClRRO-CUMULUS        ...      20,000 

f  STRATO-CIRRUS      ' 15,OCO 

MIDDLE  /  CuMUL°-CIRRUS      12,000 

"  j          Festooned  cumulo-cirrus 
Mackerel  sky 

/  STRATO-CUMULUS 6,000 

CUMULUS (base)  4,000 

1          Turretted  or  line  cumulus 
Festooned  cumulus 

LOW  I  CUMULO-NIMBUS     (base)  4,000 

"  \          Cumulo-stratus 

NIMBUS  4,500 

STRATUS      1,900 

Scud,  wrack 
\         Wreaths. 


120  WEATHER. 

Of  course  it  will  be  understood  that  the  levels  given 
here  for  Upsala  do  not  apply  to  all  the  world,  but  vary 
with  the  season  and  latitude.  They  are  introduced  here 
to  illustrate  a  great  principle,  which  holds  from  the 
equator  to  the  pole,  that  clouds  tend  to  form  at  a  few 
definite  levels,  rather  widely  separated.  Thus  at  Upsala 
we  find  the  high,  middle,  and  low  clouds  at  about  25,000, 
14,000,  and  6000  feet  respectively. 

If  we  wish  to  simplify  still  further  cloud  names  for 
observers  who  cannot  see  the  difference  between  the  ten 
principal  varieties,  we  can  put  cirro-stratus  and  strato-cirrus 
collectively  into  cirro-stratus ;  cirro-cumulus,  and  cumulo- 
cirrus  into  cirro-cumulus ;  and  treat  cumulo-nimbus  simply 
as  cumulus.  Then  we  get  only  seven  terms :  cirrus, 
cumulus,  stratus,  nimbus,  cirro-stratus,  cirro-cumulus,  and 
strato-cumulus ;  and  still  ninety  per  cent,  of  all  skies 
could  be  defined  by  these  words,  only  not  with  the  same 
precision  as  by  the  ten  varieties  before  mentioned. 

MODERN  IMPROVEMENTS. 

We  will  conclude  this  chapter  with  a  few  remarks  on 
how  far  the  increased  knowledge  of  clouds  improves  our 
capabilities  of  forecasting,  both  for  a  solitary  observer,  like 
a  man  on  board  ship,  or  for  a  central  meteorological 
bureau,  which  can  construct  synoptic  charts  by  telegraphic 
observations  from  distant  points. 

No  advance  is  more  important  than  what  we  have 
insisted  on  so  much  in  this  chapter — that  no  sky  can  be 
read  mechanically,  without  reference  to  its  surroundings. 
We  have  seen  that  there  is  fine-weather  cumulus  as  well 


CLOUDS  AND  CLOUD-PEOGNOSTICS.  121 

^as  cumulo-nimbus,  and  a  fine-weather  as  well  as  a 
dangerous  cirrus,  while  fleecy  clouds  have  not  the  same 
import  in  London  as  on  the  equator.  In  practice  the 
good  and  bad  forms  can  rarely  be  mistaken,  but  some- 
times very  difficult  cases  arise.  Clouds,  in  fact,  tell  us 
by  their  appearance,  what  might  be  written  in  words,  that 
more  or  less  damp  air  is  rising  or  falling  under  certain 
conditions  of  upper  and  lower  wind-currents.  The  signifi- 
cance must  be  judged  by  the  surroundings  and  ante- 
cedents, just  as  the  sense  of  many  words  can  only  be 
judged  by  the  context. 

Then  the  cloud-observer  who  has  added  the  modern 
knowledge  of  the  motion  of  cirrus  to  the  older  lore,  which 
is  only  concerned  with  the  kind  of  cirrus,  would  some- 
times be  able  to  indicate  weather  better  than  his  neighbours. 
And  even  if  both  would  agree  as  to  the  approach  of  rain, 
the  former  would  sometimes  be  able  to  give  much  greater 
precision  to  his  prognostications  than  the  latter. 

Another  most  important  advance  has  been  made  by 
Mr.  Ley.  He  finds  that,  like  every  other  phenomenon 
of  a  cyclone,  the  relation  of  the  upper  to  surface  winds 
is  relative  to  the  direction  in  which  the  depression  is 
moving,  and  that,  to  a  certain  extent,  the  direction  of  the 
highest  clouds  coincides  with  that  of  the  path  of  the 
cyclone.  For  instance,  if  the  cirri  in  front  of  a  cyclone 
come  from  the  south,  the  depression  will  probably  also 
advance  from  that  direction  at  some  distance  to  the  west 
of  the  observer ;  while  if  they  come  from  the  west  or 
north-west,  the  depression  will  then  most  likely  move 
from  the  westwards  also  at  some  distance  to  the  north. 
Unfortunately,  the  details  of  these  relations  are  too  com- 
plicated and  too  local  for  an  elementary  work. 


122  WEATHER. 

But  when  we  coine  to  think  how  far  cloud-observations 
may  assist  a  central  office,  the  case  is  different.  The 
foundation  of  all  modern  cloud-knowledge  turns  round  the 
relation  of  cloud-forms  to  the  shapes  of  iso baric  lines,  so 
that  though,  as  Mr.  Ley  says,  an  isolated  observer  in  the 
English  Midlands  can  plot  out  on  a  map  the  general  dis- 
tribution of  atmospheric  pressure,  and  of  weather  existing 
over  the  whole  of  the  British  Isles  at  the  time  of  his 
observations,  with  very  considerable  accuracy,  still  an 
observer  at  the  central  bureau  could  do  so  with  absolute 
accuracy.  He  could  also  often  telegraph  to  an  observer 
in  the  Midlands,  while  the  sky  was  still  cloudless  there, 
that  cirrus  of  a  particular  type  would  form  after  a  certain 
time. 

Where  cloud-observers  can  assist  a  central  office  is  in 
forecasting  that  kind  of  rain  which  is  associated  with  the 
small  secondaries  and  non-isobaric  rains  that  hardly 
show  on  synoptic  charts.  These  will  be  abundantly  dis- 
cussed in  our  chapter  on  the  subject.  For  instance, 
suppose  the  morning  reports  give  rather  ill-defined 
isobars,  with  no  rain,  but  a  good  deal  of  cloud  at  various 
stations,  the  central  forecaster  would  only  say  generally 
fine  weather,  with,  perhaps,  local  showers.  But  now,  if, 
instead  of  telegraphing  the  vague  word  "cloud,"  the 
observers  could  not  only  define  the  kind  of  cloud  accu- 
rately, but  also  give  information  as  to  the  direction  and 
velocity  of  its  motion,  relative  to  the  surface-wind,  then 
in  many  cases  the  central  office  would  see  the  incipient 
formation  of  small  rainy  secondaries,  and  the  forecasts 
sent  to  the  different  districts  would  gain  much  in  accuracy 
and  definiteness. 


PART  H. 
ADVANCED. 


(     125     ) 


CHAPTEE  IV. 

ISOBARS. 

IN  our  introduction  to  prognostics,  we  have  already  ex- 
plained the  leading  features  of  the  science  of  isobars,  and 
of  their  relation  to  the  changes  in  the  readings  of  the 
meteorological  instruments  which  are  most  usually 
observed.  In  our  chapter  on  Clouds,  we  have  also  intro- 
duced the  reader  to  the  idea  that  over  the  surface-circula- 
tion of  the  air  round  cyclones  and  anticyclones  there  is  an 
upper  circulation  of  a  very  different  character.  But  we 
must  now  go  more  deeply  into  the  subject,  so  as  to  explain 
many  details  which  could  not  then  be  conveniently  given, 
and  we  shall  not  only  complete  our  description  of  the 
nature  of  cyclones,  anticyclones,  etc.,  but  also  describe 
the  two  remaining  forms  of  isobars — Y-shaped  depressions 
and  cols — which  were  omitted  in  the  previous  chapters. 

CYCLONES. 

We  have  already  sufficiently  explained  the  broad 
features  of  a  cyclone,  and  the  wind  and  weather  which  are 
associated  with  it.  The  reader  will  now  comprehend 
what  is  meant  by  the  centre,  the  trough,  the  front  or  rear. 


126  WEATHEE. 

the  intensity,  the  path,  and  the  velocity  of  the  cyclone ; 
&nd  he  will  also  understand  that  it  is  generally  associated 
with  bad  weather,  and  rapid  shifts  of  wind  according 
to  very  definite  laws. 

What  we  want  to  consider  now  is  the  kind  of  circula- 
tion which  constitutes  a  cyclone,  and  some  points  connected 
with  the  propagation  and  motion  of  this  particular  kind 
of  low  pressure.  Conventionally,  we  shall  not  call  a  low 
pressure  a  cyclone,  unless  the  isobars  form  a  well-defined 
closed-curve.  With  the  exception  of  V-shaped  depres- 
sions, any  other  irregular  area  will  be  called  by  the 
generic  name  of  a  "  depression." 

GENERAL  CIRCULATION. 

As  the  surface-wind  in  a  cyclone  is  always  a  little 
incurved  and  the  upper  wind  always  more  or  less  out- 
curved,  the  inference  is  irresistible  that  the  main  body  of 
the  air  near  the  centre  of  a  cyclone  must  be  rising; 
otherwise,  as  the  wind  is  always  blowing  in,  the  cyclone 
would  soon  fill  up  if  there  was  no  escape  upwards.  To 
this  ascensional  movement  undoubtedly  must  be  attri- 
buted the  rain  and  cloud  which  we  find  there — rain  near 
the  centre,  where  the  ascensional  impulse  is  strongest ; 
cloud  round  the  outside,  where  the  uptake  is  less  strong. 
From  this  we  can  readily  understand  the  effect  of  what 
we  have  called  intensity  in  a  cyclone.  It  is  not  difficult  to 
conceive  a  cyclone  which  possessed  so  little  intensity 
that  it  could  only  'develop  cloud  in  the  centre.  Then, 
if  from  any  cause  the  intensity  of  the  ascensional  current 
could  be  increased,  rain  would  be  developed  where  only 


ISOBARS.  127 

cloud  had  been  formed  previously.  Thus  we  get  hold 
of  the  idea,  which  we  shall  work  out  in  some  detail  in 
future  chapters,  of  the  influences  which  can  modify  any 
existing  cyclone. 

If,  for  instance,  any  cause,  such  as  the  heating  of  the 
ground  by  the  sun,  increased  the  velocity  of  the  wind, 
and  so  poured  more  vapour-laden  air  into  the  centre  in  a 
given  time,  then  the  uptake  would  be  greater,  and  the 
tendency  to  form  rain  would  be  increased.  Similarly,  if 
the  wind  was  unchanged,  but  local  causes,  such  as  a 
range  of  hills,  gave  the  inpouring  currents  an  increased 
ascensional  impulse,  then,  too,  the  precipitation  of  rain 
would  be  still  further  developed. 

Axis. 

Returning  now  to  our  conception  of  the  cyclone  as  a 
circulatory  system,  it  is  manifest  that  we  may  consider 
the  whole  as  constituting  an  extremely  complicated 
vortex,  something  analogous  to  an  eddy  of  water.  There 
is,  however,  this  difference — that  a  water-eddy  sucks 
down,  while  an  aerial  cyclone  draws  upwards. 

The  line  along  which  any  particle  of  air  may  be 
supposed  to  move  must  not  only  curve  irregularly  inwards, 
but  also  upward,  and  finally  outwards.  As  the  whole  is 
treated  as  a  whirling  system,  there  must  be  a  line,  "more 
or  less  perpendicular  to  the  earth's  surface,  round  which 
the  air  rotates  in  this  complicated  manner.  This  imaginary 
line  is  called  the  axis  of  the  cyclone. 

There  is  much  uncertainty  as  to  the  nature  of  the  cir- 
culation round  this  axis.  Some  writers  have  thought  of 


128  WEATHER. 

the  axis  of  a  top,  and  believed  that  the  axis  of  a  cyclone 
can  nutate,  always  keeping,  a  revolving  disc  of  air  per- 
pendicular to  itself,  so  that  the  cyclone  would  be  pressed 
down  on  the  ground  in  the  direction  towards  which  the 
axis  inclined,  and  be  lifted  off,  as  it  were,  on  the  opposite 
side.  This,  they  say,  would  explain  the  anomalies  that  are 
sometimes  found  both  in  the  position  of  steepest  gradients 
relative  to  the  centre,  and  in  the  variable  .destructiveness 
of  the  wind. 

With  the  same  velocity,  wind  will  sometimes  unroof 
houses,  at  other  times  do  little  damage ;  and  they  consider 
that  in  the  first  case  the  direction  of  the  wind  is  a  little 
upwards,  in  the  latter  a  little  downwards.  They  believe 
that  this  conception  of  an  inclined  axis  is  confirmed  by 
the  fact  that,  in  tropical  cyclones,  the  small,  clear  patch 
of  blue  sky  in  the  centre  of  a  cyclone  is  not  always 
exactly  over  the  point  of  lowest  barometer.  They  would 
then  consider  that  the  axis  of  the  cyclone  is  like  a 
telescope  pointed  upwards,  at  some  angle  from  the  ground, 
instead  of  truly  vertical. 

The  insuperable  difficulty  in  the  way  of  all  this  lies  in 
the  fact  that  a  cyclone  is  often  one  or  two  thousand  miles 
across,  and  certainly  not  more  than  ten  miles  deep ;  so 
that  the  amount  of  tilt  required  to  give  the  observed 
deflection  of  isobars  would  be  sometimes  20°  or  more, 
and  that,  with  a  disc  of  two  thousand  miles,  would  be 
impossible  under  the  conditions  of  our  earth. 

If,  for  instance,  we  glance  back  for  a  moment  to  our 
typical  cyclone  (Fig.  2),  we  see  that  the  isobar  of  29  ins. 
is  not  concentric  with  that  of  30  ins.  The  idea  would  be 
that  a  vertical-axis  cyclone  would  have  concentric  circular 


ISOBARS.  129 

isobars,  but  that  both  the  oval  form  and  setting  back 
of  the  isobars  at  one  side  are  due  to  the  axis  being  tilted 
forward  towards  the  word  Front. 

Another  theory  to  account  for  all  these  facts  supposes 
that  a  cyclone  is  made  of  a  series  of  flat  oval  horizontal 
sections,  but  that  these  are  not  superimposed  concentrically 
one  on  the  top  of  the  other,  but  pressed  successively  more 
or  less  to  one  side  by  surrounding  influences.  In  Fig.  2 
this  shunt  would  have  been  towards  the  rear,  instead  of 
forwards  as  by  the  preceding  hypothesis. 

This  view  is  probably  partially  correct,  though  it  is 
impossible  to  suppose  that  the  air  does  not  get  more  or 
less  inclined  upwards  at  times,  for  no  cyclone  is  ever 
absolutely  symmetrical. 

We  often  see  the  conical  revolving  cloud  of  a  whirl- 
wind or  tornado  bending  about  like  the  trunk  of  an 
elephant,  with  both  a  true  axial  inclination  and  a  certain 
amount  of  sideways  shunt.  Here,  however,  the  vertical 
height  is  enormously  greater  than  the  diameter,  which  is 
just  the  opposite  to  the  proportions  of  a  cyclone.  Anyhow, 
if  we  suppose  that  upper  winds  follow  the  same  laws  as 
surface-currents  with  relation  to  isobars,  observations  on 
cirrus-clouds  tend  to  the  belief  that  the  axis  of  a  cyclone 
is  very  often  inclined  backwards  from  the  direction  in 
which  the  cyclone  is  moving,  as  if  the  surface  portion  was 
going  faster  than  the  upper. 

This  is  just  the  converse  of  what  might  have  been 
expected  a  priori,  that  surface  friction  would  retard  the 
lower  portions,  so  that  the  axis  of  the  cyclone  would  have 
been  inclined  forwards.  The  whole  question  is,  however, 
still  very  obscure. 


130  WEATHER. 

PEOPAGATION. 

When  we  come  to  consider  the  nature  of  the  propaga- 
tion of  a  cyclone,  we  are  met  with  many  difficulties.  At 
first  it  might  be  thought  that  a  cyclone  could  be  treated 
as  a  rotating  disc,  which  was  impelled  along  the  earth's 
surface  by  some  force ;  but  there  are  conclusive  reasons 
against  such  a  supposition.  If  this  was  really  the  case, 
we  ought  to  be  able  to  compound  the  rotation  and  trans- 
lation motions  of  any  particle  of  air  in  the  usual  manner ; 
but  when  we  do  so,  we  find  that  we  get  winds  very 
different  from  what  are  actually  observed.  Take  that 
portion  of  the  front  of  a  cyclone  where  the  wind  from 
rotation  would  be  south,  and  suppose  it  to  be  compounded 
with  even  a  slow  motion  of  translation  towards  the  west, 
then  the  resulting  wind  must  have  a  slant  from  tlie  west 
— that  is  to  say,  it  would  blow  outwards  in  front  of  the 
centre.  Now,  this  is  exactly  what  it  does  not  do.  Observa- 
tion shows  that  the  wind  is  more  incurved  in  front  of  a 
cyclone  than  in  any  other  portion,  and  therefore  the  idea 
of  a  rotating  disc  cannot  be  maintained.  We  are  almost 
compelled  to  believe  that  a  cyclone-vortex  is  propagated 
in  a  manner  somewhat  analogous  to  a  wave  of  water. 
When  a  wave  approaches  the  shore,  the  first  impulse  is 
always  an  indraught,  though,  of  course,  the  motion  of  the 
wave  is  forwards ;  and  when  a  cyclone  approaches,  the 
first  impulse  is  likewise  inwards.  Here,  however,  the 
analogy  probably  ceases. 

Another  analogy  to  vortex-motion  is  found  in  the 
manner  in  which  a  cyclone,  as  a  whole,  is  deflected  by 
areas  of  high  pressure. 


ISOBARS.  131 

We  know  by  experiment  that  a  vortex-ring  of  smoke 
has  great  stability,  and  can  be  twisted  and  deflected  like 
an  elastic  body.  Similarly  we  see  a  cyclone  move  up 
against  an  area  of  high  pressure  and  be  rebuffed  from  it, 
even  though  the  cyclone  may  be  one  thousand  or  more 
miles  in  diameter.  Of  course,  it  must  be  understood  that 
in  a  vortex-ring  each  particle  of  air  revolves  in  a  complete 
circle,  while  in  a  cyclone  any  particle  hardly  describes  a 
semicircle,  so  that  the  analogy  is  only  very  partial. 

STABILITY. 

However,  from  this  conception  of  a  stable  vortex  we 
can  understand  what  has  long  been  a  puzzle  to  meteor- 
ologists— why  great  changes  of  temperature  between 
night  and  day  can  be  associated  with  fine  weather. 

Differences  in  temperature  between  two  adjacent  areas 
have  always  been  supposed  to  set  up  currents  of  air,  and 
from  this  it  has  been  thought  that  weather-changes  could 
be  deduced.  In  practice,  however,  it  is  far  otherwise. 
In  some  of  the  most  settled  climates  in  the  world,  such  as 
Persia  or  Northern  Africa,  the  difference  between  the  day 
and  night  temperatures  is  often  from  30°  to  40°  Fahr. ; 
while  in  unsettled  climates,  like  Great  Britain,  10°  Fahr. 
(5°  C.)  is  a  large  amount. 

Then  consider  that  a  difference  of  16°  Fahr.  (9°  C.) 
makes  as  much  change  in  the  density  or  specific  gravity 
of  a  cubic  foot  of  air  as  a  change  of  one  inch  of  mercurial 
pressure,  and  we  may  well  wonder  why  such  great  changes 
have  so  little  effect  on  weather. 

But  when  we  know  that  a  cyclone  and,  as  we  shall 


132  WEATHER. 

afterwards  see,  an  anticyclone  are  both  to  a  certain  extent 
vortices,  and  that  all  the  world  is  generally  covered  by 
one  or  the  other,  then  we  can  readily  understand  that  as 
such  they  have  great  stability,  and  that,  though  changes 
of  temperature  might  affect  the  velocity  or  direction  of 
the  general  circulation,  they  could  not  break  up  or  destroy 
any  existing  system.  In  fact,  the  atmosphere  is  not  an 
inert  mass,  ready  to  be  swayed  by  any  trifling  disturbance, 
but  is  always  broken  up  into  circulatory  systems,  which 
possess  a  very  considerable  amount  of  stability.  When 
we  come  to  discuss  diurnal  variations  of  meteorological 
elements,  we  shall  find  that  the  difference  of  day  and 
night  temperatures  only  impose  a  very  small  modifica- 
tion on  the  general  character  of  the  weather. 

INFLUENCE  OF  KAINFALL  AND  TEMPERATURE. 

There  are  two  other  points  connected  with  the  origin 
and  motion  of  cyclones  which  need  only  be  alluded  to 
in  an  elementary  work  like  the  present — the  influence  of 
rainfall,  and  the  surrounding  distribution  of  temperature. 
In  most  cases  the  greater  portion  of  the  rain  falls  in  front 
of  the  cyclone's  centre,  or  trough,  and  it  used  to  be  thought 
that,  the  rain  being  produced  by  some  unknown  cause, 
the  cyclone  ran  after  the  vacuum  which  was  left  by  the 
condensation  of  vapour.  It  was,  moreover,  believed  by 
some  that  this  condensation  of  vapour  was  also  the  origin 
of  the  cyclone.  Air  was  supposed  to  rush  into  the 
vacuum,  to  pick  up  circulation  through  the  influence  of 
the  earth's  rotation,  and  thus  to  form  an  eddy. 

Later  observations  have,  however,  completely  disproved 


ISOBARS.  133 

both  these  views.  Under  certain  conditions  of  surround- 
ing pressure,  the  greatest  rainfall  in  a  cyclone  occurs  in 
rear  of  the  centre  ;  and  the  commencement  of  a  cyclone 
is  neither  always  preceded  by  rain,  nor  does  the  depth  of 
a  cyclone  bear  any  relation  whatever  to  the  amount  of 
precipitation. 

In  practice  the  heaviest  rain  is  with  the  slight 
gradients  of  a  secondary,  and  in  very  heavy  showers  the 
barometer  generally  rises. 

The  general  view  we  wish  to  present  is  that  of  the  air 
rising  over  a  hot  equator,  and  pouring  down  towards  the 
pole  by  its  own  weight.  Mere  irregularities  of  this  down- 
flow  would  be  sufficient  to  form  eddies  without  any  con- 
densation of  vapour  to  cause  vacuum,  and  develop  heat, 
as  we  can  readily  prove  by  experimenting  with  water  or 
smoke. 

If  the  atmosphere  was  absolutely  vapourless,  and  the 
sun  moved  round  a  motionless  earth,  the  irregular  over- 
flow of  air  would  form  eddies  of  some  sort,  very  different 
from  those  we  now  know.  If  the  same  vapourless  atmo- 
sphere rotated  with  the  earth,  round  a  stationary  sun,  as 
at  present,  cloudless  cyclones  and  anticyclones  would 
undoubtedly  form,  perhaps  not  so  very  different  from 
those  we  know  now ;  while  the  addition  of  water- vapour 
would  make  the  rotational  systems  precisely  such  as  we 
now  observe. 

Though  heat  and  vapour  do  play  a  considerable  part 
in  the  mechanism  of  a  cyclone,  we  shall  abundantly  show, 
in  the  course  of  this  work,  that  cyclones  are  only  incidents, 
as  it  were,  in  greater  movements  of  the  atmosphere,  and 
that  most  temperature-changes  are  due  to  differences  of 


WEATHER. 

radiation,  caused  by  the  wind,  calm,  cloud,  or  blue  sky 
associated  with  different  kinds  of  aerial  eddies. 

An  enormous  amount  has  been  written  in  Germany, 
India,  and  the  United  States  on  the  influence  of  surround- 
ing temperature,  and  of  the  latent  heat  released  by  the 
condensation  of  vapour  into  rain  in  determining  the  course 
of  a  cyclone.  Most  of  this  is,  however,  too  much  mixed  up 
with  theory,  and  too  much  encumbered  with  mathematical 
formulae,  to  find  a  place  in  this  work.  There  is,  unfor- 
tunately, no  consecutive  account  of  those  investigations  in 
the  English  language,  except  certain  papers  of  Professor 
Ferrel,  published  by  the  Signal  Office  in  Washington, 
which  cannot  be  purchased;  but  they  are  admirably 
given  in  Sprung,  "  Lehrbuck  der  Meteorologie,"  and 
J.  von  Bebber,  "  Handbuch  der  ausubenden  Witterungs- 
kunde." 

We  shall  recur  to  this  subject  in  our  chapter  on  Fore- 
casting by  Synoptic  Charts,  as  by  that  time  we  shall  be 
better  able  to  understand  the  nature  and  surroundings  of 
a  cyclone. 

Our  knowledge  of  the  nature  of  cyclones  would  be 
much  more  complete  if  we  knew  how  high  in  the  atmo- 
sphere they  extended.  In  thunderstorms  we  often  get  a 
complete  circulation  of  the  wind  on  the  surface,  while  the 
upper  currents  retain  their  direction  unchanged,  and  the 
drift  of  the  storm  is  usually  with  this  higher  wind.  Un- 
fortunately, we  cannot  trace  a  continuous  development  of 
these  small  circulations  into  regular  cyclones. 


ISOBARS.  135 

TROPICAL  AND  EXTRA-TROPICAL  CYCLONES. 

It  has  been  too  much  the  custom  in  meteorological 
books  to  treat  tropical  cyclones  apart  from  similar  dis- 
turbances in  extra-tropical  or  temperate  regions.  We 
have  made  numerous  researches  on  the  subject  in  India, 
the  China  Seas,  Japan,  and  Mauritius,  and  found  that, 
though  the  general  character  of  all  cyclones  is  the  same, 
there  are  differences  of  detail  which  throw  an  immense 
amount  of  light  on  the  cause  of  the  great  variety  in  the 
appearance  of  the  sky  in  different  parts  of  the  same 
cyclone. 

All  cyclones  agree  in  the  great  features  of  the  wind 
rotating  round  the  centre  with  a  variable  indraught,  and 
of  an  upward  and  outward  circulation  of  the  higher 
currents. 

No  more  conclusive  proof  of  this  can  be  found  than 
the  fact  that  cyclones  often  pass  out  of  the  tropics,  and 
then  join  or  coalesce  with  others  which  have  been  formed 
without  the  tropics.  Two  similar  eddies  can  easily  unite, 
but  two  that  rotated  on  different  systems  would  infallibly 
break  each  other  up. 

The  typical  cyclone  in  all  parts  of  the  world  is  certainly 
oval,  with  the  inner  isobars  usually  closer  to  the  rear  than 
to  the  front ;  and  the  rain  extends  further  before  than 
behind  the  trough.  But  the  tropical  cyclone  has  a  striking 
feature  which  is  absent  in  our  latitudes.  There  is  a 
patch  of  blue  sky  over  the  calm  centre,  which  is  well 
known  in  hurricane  countries  as  the  "  eye  of  the  storm," 
or  as  a  "bull's-eye."  Then  cirrus  and  halo  appear  all 
round  a  tropical  cyclone,  while  they  are  never  seen  in 


136  WEATHER. 

rear  of  a  European  storm  ;  and  though  the  way  in  which 
the  rain  seems  to  grow  out  of  the  air  in  front  of  a  cyclone 
is  the  same  everywhere,  the  sky  and  clouds  in  rear  of  a 
hurricane  are  much  softer  and  dirtier  than  in  temperate 
cyclones.  There  is  not  that  sharp  difference  between  the 
quality  of  clouds  in  front  and  rear  which  is  so  striking 
in  higher  latitudes.  Still  greater  is  the  absence  of  any 
marked  squall  or  change  of  weather  during  the  passage  of 
the  trough  in  the  tropics — that  is,  at  the  moment  when 
the  barometer  begins  to  turn  upwards.  Some  who  study 
hurricanes  have  scarcely  noticed  any  change  then ;  and 
all  are  agreed  that  the  trough-phenomena  are  very  slight. 

We  have  already  shown,  in  our  chapter  on  Prog- 
nostics, that  a  cyclone  has,  as  it  were,  a  double  symmetry. 
One  set  of  phenomena,  such  as  wind,  cloud,  and  rain,  are 
grouped  round  the  centre  ;  while  the  second  set,  such  as 
the  different  character  of  the  heat  and  clouds  in  front 
and  rear,  and  the  line  of  squalls  along  the  line  where  the 
barometer  begins  to  rise,  are  related  to  the  trough  of 
the  cyclone.  If  we  call  the  first  set  the  rotational,  and 
the  second  set  the  translational,  phenomena  of  a  cyclone, 
we  find  that  the  former  are  all  more  marked  in  the 
tropical,  and  the  latter  in  extra-tropical,  cyclones.  Then, 
if  we  examine  the  charts  of  cyclones,  we  see  that,  while 
tropical  hurricanes  are  much  smaller,  and  have  much 
stronger  winds  than  any  others,  they  only  move  from  two 
to  ten  miles  an  hour ;  while  extra-tropical  cyclones  rotate 
much  more  slowly,  but  are  propagated  at  a  rate  of  from 
twenty  to  seventy  miles  an  hour. 

Thus  we  might  readily  suppose  that  what  we  call 
rotational  phenomena  are  really  due  to  the  circulation, 


ISOBARS.  137 

and  the  translational  phenomena  to  the  forward  motion,  of 
a  cyclone ;  and  we  are  confirmed  in  this  view  by  an 
examination  of  Japanese  typhons.  That  semi-tropical 
country  is  traversed  by  cyclones  of  two  different  types  at 
different  seasons  of  the  year,  that  move  with  different 
velocities,  and  they  find  that  all  the  trough-phenomena 
are  more  marked  in  the  quickly  moving  cyclones  than  in 
those  whose  progress  is  slower. 

These  researches  also  lead  to  another  most  important 
conclusion — that  the  character  of  cloud  and  weather 
depends  on  the  position  relative  to  the  front  of  a  cyclone, 
and  not  on  the  direction  of  the  wind.  Cyclones  in 
Europe  move  towards  the  east,  and  the  dirty  sky  comes 
with  a  south-east  wind ;  while  in  the  northern  tropics 
hurricanes  move  towards  the  west,  and  the  same  sky 
comes  with  a  north-west  wind.  People  sometimes  say 
that  of  course  the  rear  of  a  cyclone  must  be  clear,  because 
of  a  cold,  dry  north-west  wind  ;  but  when  a  cyclone  moves 
west,  even  in  Europe,  that  wind  becomes  close  and 
dirty. 

We  shall  defer  our  consideration  of  cyclones  in  the 
southern  hemisphere  till  our  chapter  on  Winds,  because 
it  is  only  the  direction  of  the  wind,  and  not  the  sequence 
of  weather,  which  is  altered  in  comparison  with  northern 
storm-systems. 

ANTICYCLONES. 

If  we  turn  to  the  diagram  (Fig.  16)  of  surface  and 
upper  winds  in  an  anticyclone  which  we  gave  in  our 
chapter  on  Clouds,  we  shall  see  at  once  that  it  presents 


138  WEATHER. 

some  analogies,  as  well  as  some  very  striking  contrasts,  to 
the  cyclone  figure.  The  anticyclone  blows  round  and 
out  below,  round  and  in  above,  and  therefore  the  con- 
clusion is  obvious  that  the  air  in  the  centre  of  an  anti- 
cyclone must  be  descending.  It  must,  then,  necessarily 
be  unusually  dry,  and  this  is  just  what  observation 
shows  it  is.  Then  exactly  the  same  argument  holds  as  in 
a  cyclone,  that,  as  a  whole,  an  anticyclone  is  a  complex 
vortical  system  which  possesses  so  much  stability  that 
great  diurnal  changes  of  temperature  do  not  affect  it  as  a 
whole. 

It  may  be  well  to  note  the  higher  character  of  the 
explanation  of  weather  which  we  can  give  now,  as  com- 
pared to  what  we  said  when  treating  of  prognostics. 
Then  we  merely  said  that,  as  a  matter  of  blind  obser- 
vation, the  centre  of  a  cyclone  was  rainy,  and  that  of  an 
anticyclone  bright.  Now  we  show  that  these  two  varieties 
of  weather  are  the  necessary  product  of  different  kinds  of 
atmospheric  eddies. 


PRESSURE  OVER  CYCLONES  AND  ANTICYCLONES. 

Simultaneous  observations  at  the  top  and  bottom  of 
high  mountains  have  demonstrated  that  the  difference  of 
pressure  for  a  given  height  is  always  less  in  cyclones  than 
in  anticyclones;  also  that  the  fall  of  the  barometer  is 
always  less  pronounced  on  the  summit  than  at  the  base 
of  a  mountain.  For  instance,  if  the  difference  of  pressure 
between  a  high  and  low  level  station  was  four  inches  in 
a  cyclone,  it  might  be  four  and  a  quarter  in  an  anti- 
cyclone ;  and  if  the  barometer  fell  an  inch  at  the  sea-level, 


ISOBARS. 


139 


the  fall  might  only  be  about  eight-tenths  of  an  inch  on 
the  top  of  a  mountain  five  thousand  feet  high. 

The  inference  which  is  drawn  from  this  is  that,  as  we 
ascend,  the  gradients  between  a  cyclone  and  its  adjacent 
anticyclone  must  diminish,  and  it  is  by  no  means  im- 
probable that  if  we  went  up  high  enough  we  should  find 
them  inverted ;  that  is  to  say,  that  the  higher  pressure 
would  be  over  the  cyclone. 

In   Fiof.  20  we  have  drawn  an  ideal  sketch  of  the 


*3 


Spoo.  _ 


31 


Cyclone,  Anticyclone. 

FIG.  20. — Probable  vertical  gradients  over  cyclone  and  anticyclone. 

probable  so-called  vertical  gradients  over  a  cyclone  and 
its  adjacent  anticyclone.  The  line  at  the  bottom  repre- 
sents the  level  of  the  earth's  surface.  If  the  pressure  is 
31  ins.  over  the  anticyclone,  and  only  27  ins.  over  the 
cyclone,  the  vertical  isobar  of  27  ins.  must  be  as  we  have 
drawn  it.  But,  as  pressure  decreases  more  rapidly  over  an 
anticyclone,  it  seems  probable  that  at  a  certain  level — 


140  WEATHER. 

which  we  have  assumed  here  as  ten  thousand  feet — there 
would  be  no  gradient  either  way  ;  and  that  still  higher 
up  the  pressure  would  be  actually  greater  over  the  cyclone, 
and  the  gradient  inverted  as  compared  to  that  on  the 
surface.  It  is  evident,  therefore,  that,  though  we  may 
use  the  analogy  of  cyclones  to  whirlpools,  we  must  not 
picture  the  former  to  ourselves  as  saucer-shaped  depres- 
sions of  the  whole  envelope  of  our  atmosphere,  like  the 
little  eddies  that  pit  the  surface  of  a  flowing  river.  The 
important  bearing  of  these  vertical  gradients  on  the  pro- 
blem of  measuring  heights  by  the  barometer  will  be 
very  obvious. 

This  brings  us  to  a  curious  question,  as  to  what 
gradients  give  direction  to  the  upper  winds.  Some  have 
maintained  that  the  uprushing  currents  of  a  cyclone  have 
so  much  momentum  that  they  can  override  moderate 
gradients.  Others,  on  the  contrary,  hold  that  it  is  only 
by  a  complete  or  partial  inversion  at  high  levels  of  the 
gradients  which  are  found  on  the  surface,  that  the  observed 
phenomena  of  upper  currents  can  be  produced;  but 
materials  do  not  at  present  exist  which  can  decide  the 
question,  and  some  of  the  published  sketches  of  the 
higher  isobars  over  a  cyclone  are  more  than  problematical. 

Another  point  on  which  considerable  uncertainty 
exists  is  the  relation  of  anticyclones  to  cyclones.  There 
is  no  doubt  of  their  partial  dependence  on  one  another, 
for  cyclones  always  tend  to  travel  round  the  anticyclone 
on  whose  edge  they  have  been  formed.  But,  on  the  other 
hand,  we  sometimes  find  cyclones  which  have  detached 
themselves  from  their  generating  anticyclones,  and  this 
seems  to  prove  that  they  can  exist  independently. 


ISOBARS.  141 

ANTITHESIS  OF  CYCLONIC  AND  ANTICYCLONIC  WEATHER. 

Perhaps  the  best  method  of  showing  the  antithesis 
between  cyclone  and  anticyclone  weather  will  be  the 
method  we  have  adopted  in  Figs.  21  and  22.  We  all 
know  how  much  weather  is  affected  by  the  time  of  day, 
as  well  as  by  the  season  of  the  year,  and  by  local  pecu- 
liarities. We  have,  therefore,  selected  two  charts  for  the 
same  day  of  different  years,  at  the  same  hour  of  the 
morning,  and  for  the  same  portion  of  Western  Europe. 
Every  diurnal,  seasonal,  or  local  influence  is  therefore 
identical  in  both  cases,  and  the  whole  of  the  difference  of 
wind  and  weather  which  we  find  between  the  two  days  is 
entirely  due  to  cyclonic  or  anticyclonic  influences. 

In  Fig.  21  we  give  the  synoptic  conditions  of  pressure, 
temperature,  wind,  and  weather  over  Western  Europe  at 
8  a.m.,  May  17,  1877.  There  we  see  a  small  oval  cyclone 
of  very  moderate  intensity  lying  over  the  south-west  of 
England.  Bound  this  the  wind  circulates  in  the  usual 
manner,  but,  as  the  gradients  are  not  steep,  the  force 
nowhere  exceeds  a  fresh  breeze,  as  at  Brest.  Near  the 
centre,  and  some  distance  in  front,  we  find,  by  looking  at 
the  weather-symbols,  nothing  but  rain  reported ;  outside 
the  rain,  an  overcast  sky  or  detached  clouds ;  and  beyond 
them,  blue  sky  in  a  few  places.  The  path  of  the  cyclone 
is  marked  by  the  letters  &,  x,  so  as  to  give  the  position  of 
the  front.  Lastly,  the  isotherm  of  60°  Fahr.  (16°  C.)  runs 
just  north  of  the  Pyrenees,  while  that  of  50°  Fahr.  (10°  C.) 
stretches  from  the  north  of  Scotland  to  Denmark. 

Now  turn  to  Fig.  22,  where  we  give  the  same  data  for 
May  17,  1874,  at  the  same  hour.  Then  an  anticyclone 


142 


WEATHER. 


lay  over  the  British  Islands ;  the  gradients  were  much 
less  steep,  and  the  wind,  therefore,  was  everywhere  light 
and  variable.  For  this  reason  the  general  circulation  is 
not  so  marked  as  in  the  preceding  chart,  but  still  it  is 
very  evident  that  on  the  whole  the  wind  blew  round  and 
out  in  the  direction  of  the  watch-hands.  Then  the  weather- 
symbols  are  very  interesting.  Almost  every  station  which 


x7-5-77 
8. a.m. 


50 


17-5-74 
8. a.m. 


FIG.  21. — Cyclone  weather. 


FIG.  22. — Anticyclone  weather. 

reported  rain  in  the  previous  chart  is  now  marked  with 
b  for  blue  sky,  or  with  m  for  radiation  mist  in  several 
places.  Then,  though,  as  at  Fano,  in  Denmark,  we  find 
the  same  symbol,  c,  for  detached  cloud  in  both  maps,  we 
know  that  it  does  not  refer  to  the  same  kind  of  cloud. 
The  temperature  also  shows  a  marked  contrast.  The 
isotherm  of  60°  has  not  much  altered  its  position,  but 


ISOBARS.  143 

that  of  50°  Fahr.  (10°  C.)  bends  abruptly  south  from  the 
north  of  Scotland,  across  England,  and  the  west  of  France, 
while  temperatures  below  40°  (5°  C.)  are  reported  from 
Hanover  and  the  Netherlands.  This  is  partly  due  to  the 
prevailing  set  of  the  wind  being  from  the  north,  whereas 
in  the  preceding  chart  it  was  from  the  south  or  south- 
west. 

From  these  examples,  we  see  that  the  whole  of  the 
difference  of  weather  on  the  two  days  was  produced  by 
the  difference  of  isobars,  which  we  may  put  thus : 

The  weather  was  wet  on  May  17,  1877,  over  England, 
because  then  the  atmosphere  was  eddying  in  that  manner 
which  we  call  cyclonic ;  while  it  was  fine  on  May  17,  1874, 
because  then  the  eddy-circulation  took  the  form  which  we 
call  anticyclonic,  and  drew  down  dry  air  from  the  upper 
regions  of  the  atmosphere. 

V-SHAPED  DEPRESSIONS. 

We  must  now  describe  a  very  interesting  shape  of 
isobars,  to  which  the  name  of  V-depressions  is  applied  in 
England,  but  which  the  German  writers  call  "tongue- 
formed"  depressions.  In  these  the  isobars  are  shaped 
like  the  letter  V,  and  enclose  an  area  of  low  pressure.  In 
the  northern  hemisphere  the  point  of  the  V  is  usually 
directed  towards  the  south,  as  in  Fig.  23.  The  wind 
follows  the  universal  law  of  gradients ;  being  from  south 
to  south-west  in  front,  and  from  west  to  north-west  in 
rear  of  the  trough.  This  latter  line  is  given  at  once  by 
joining  the  southern  points  of  each  successive  isobar,  and 
in  practice  is  nearly  always  curved,  the  convexity  being 


144 


WEATHER. 


turned  towards  the  east,  as  in  the  diagram.  As  the  V  is 
usually  moving  towards  the  east,  this  line  marks  out  the 
position  of  all  the  places  at  which  the  barometer,  having 
fallen  more  or  less,  has  just  turned  to  rise,  and  is  called 
the  "  trough  "  of  the  V. 

These  features  are  common  to  all  V's,  but  the  position 
of  rain  divides  these  depressions  into  two  distinct  types. 

In  the  first,  and  by  far  the 
commoner  kind  in  Great 
Britain,  a  narrow  strip  of  cloud 
precedes  an  area  of  rain, 
shaped  like  a  portion  of  a 
crescent.  This  is  shown  in 
Fig.  23,  where  the  single 
shading  marks  the  position 
and  shape  of  the  cloud-area, 
and  the  double  shading  that 
of  the  rain.  The  rear  of  the 
rain-area  is  very  sharply  de- 
fined by  the  line  of  the  trough, 
which  also  marks  the  position 
of  a  line  of  squalls.  Beyond 
this  we  find  detached  clouds, 
and  then  blue  sky. 

The  sequence  of  weather, 
as  a  V  of  this  type  drifts 
over  an  observer,  is  obviously  from  blue  sky  to  halo, 
cloud,  and,  later  on,  rain,  with  a  falling  barometer  and 
south-west  wind ;  then  a  heavy  squall,  during  which  the 
wind  jumps  (does  not  veer)  to  north-west,  and  the  sky 
rapidly  clears  as  the  barometer  rises. 


Blue 


Sky 


FIG.  23.— Weather  in  Y-de- 
pression. 


ISOBARS.  145 

In  the  other  kind,  which  is  less  common,  the  front  of 
the  V  is  cloudy,  but  half  a  crescent-shaped  area  of  rain  is 
formed  in  the  rear.  The  front  of  this  area  is  sharply 
defined  by  the  trough,  while  the  rear  tails  off  through 
cloud  to  blue  sky.  An  illustration  of  this  type  will  be 
found  in  Figs.  49,  50,  and  51,  in  our  chapter  on  Squalls. 

The  sequence  of  weather  to  a  solitary  observer  will 
be  a  falling  barometer,  with  a  cloudy  sky  and  wind  from 
the  south-west.  A  heavy  bank  of  cloud  approaches  from, 
the  north-west ;  this  passes  over  him  with  a  heavy  squall, 
the  wind  jumps  to  north-west,  and  the  mercury  turns 
upwards.  After  the  first  violence  of  the  squall  is  over, 
driving  rain  continues  for  some  time,  and  dies  away 
gently  as  the  sky  becomes  clear  again. 

This  class  of  V  is  usually  followed  by  a  second  depres- 
sion of  some  sort,  as  will  be  seen  by  Fig.  49  in  our  chapter 
on  Squalls.  There  seems  to  be  some  connection  between 
the  two  depressions  and  the  area  of  high  pressure  between 
them,  but  the  subject  has  not  yet  been  worked  out. 

V's  are  generally  formed  either  along  the  southern 
prolongation  of  the  trough  of  a  cyclone,  or  else  in  the  col 
between  two  adjacent  anticyclones ;  and  a  V  of  the  fii  st 
type  bears  the  same  relation  to  a  wedge  that  a  cyclone 
does  to  an  anticyclone  ;  that  is  to  say,  one  is  as  nearly  as 
possible  the  converse  of  the  other.  The  most  interesting 
feature  about  V's  is  their  relation  to  cyclones ;  for,  while 
a  cyclone  has,  as  it  were,  a  double  symmetry — namely,  one 
set  of  phenomena,  such  as  temperature  and  the  kind  of 
cloud  symmetrically  disposed  in  front  and  rear  of  the 
trough,  and  another  set,  such  as  wind  and  rain,  symmetri- 
cally arranged  round  the  centre — a  V-shaped  depression 


146  WEATHER. 

has  only  one  line  of  symmetry — the  trough — to  the  front 
and  rear  of  which  alone  both  wind  and  weather  are  related. 
When  we  consider  that  in  extra-tropical  cyclones,  though 
the  isobars  are  circular,  the  wind  makes  a  sudden  shift  as 
the  trough  passes,  and  that  V's  have  nothing  cyclonic  at 
all  about  them,  we  can  readily  understand  the  difficulties 
which  many  felt  in  accepting  the  theory  of  cyclones,  and 
how  at  first  sight  it  appears  much  simpler  to  assume  the 
conflict  of  two  opposing  currents,  rather  than  the  circula- 
tion of  a  great  eddy. 

Though  a  cyclone  is  a  edcly,  the  opposite  properties 
of  its  front  and  rear" do  not  suggest  true  circulation,  while 
a  V  is  something  of  a  totally  different  nature.  The  error 
consisted  in  assuming  that  the  bad  weather  of  the  V  was 
of  the  same  nature  as  that  in  a  true  cyclone.  There  is 
Very  great  difficulty  in  forming  a  rational  conception  of 
the  general  circulation  in  a  V.  The  currents  in  front  and 
rear  of  the  trough  are  not  exactly  opposite,  and  we  know 
nothing  of  the  motion  of  the  upper  currents,  so  that  we 
must  await  the  results  of  future  research. 

SOUTHERLY  BURSTERS. 

V-depressions  are  not  at  all  common  in  the  tropics, 
though  we  have  observed  a  squall  of  this  type  near  New 
Caledonia,  in  about  lat.  22°  south  ;  but  we  have  discovered 
that  a  great  many  of  the  so-called  "  southerly  bursters  " 
off  Australia  are  due  to  the  class  of  Y's  in  which  rain 
falls  in  rear  of  the  trough. 

The  point  of  a  V  in  the  southern  hemisphere  is 
pointed  towards  the  north,  while  the  wind  is  north-east 


ISOBARS.  147 

or  north  in  front,  and  south-west  or  south  in  rear  of  the 
trough  (see  Fig.  39,  p.  199). 

The  first  blows  in  Australia  across  a  burning  continent 
of  hot  sand,  and  as  the  gusts  increase  near  the  trough, 
clouds  of  suffocating  dust  are  added  to  the  furnace-like 
heat  till  existence  is  almost  unbearable.  Then  suddenly 
the  wind  jumps  round  to  south-west  or  south,  and  heavy 
rain  is  driven  before  the  chilly  blasts  from  the  ice-bound 
southern  pole.  Temperature  occasionally  falls  30°  or  40° 
in  a  single  hour.  Sometimes  the  "  burster  "  is  associated 
with  the  very  similar  sequence  of  weather  during  the 
passage  of  the  trough  of  a  cyclone. 

COLS. 

The  last  shape  of  isobars  which  we  have  to  describe  is 
the  "  col,"  or  neck  of  low  pressure,  which  lies  between  two 
adjacent  anticyclones.  We  will  describe  a  specimen  of 
the  most  common  case  of  European  col.  We  find  in 
Eig.  24  that  a  portion  of  one  anticyclone  lies  over  the 
Bay  of  Biscay,  while  another  lies  to  the  north-east  of  this 
over  the  Scandinavian  peninsula.  Then,  while  one  area 
of  comparatively  low  pressure  lies  to  the  north-west  of 
Iceland,  another  covers  Central  Europe  and  the  north  of 
Italy,  so  that  a  saddle-back  area,  or  neck,  of  low  pressure 
is  found  over  England.  In  the  middle  of  the  col  there  is 
no  gradient,  and  therefore  a  calm,  while  all  round  the 
winds  and  weather  conform  to  the  usual  law  of  isobars. 
The  weather  is  dull,  gloomy,  and  stagnant,  while  in 
summer  violent  thunderstorms  are  frequently  found  in 
different  portions  of  a  col. 


148 


WEATHEE. 


20.4.83. 


As  a  whole  no  sequence  of  weather  can  be  assigned  to 
a  col.  It  does  not  move  itself,  but  no  law  can  be  laid  down 
to  say  whether  the  col  will  remain  stationary,  or  whether 

the  area  which  it  covers  to-day 
will  be  occupied  by  some 
other  type  of  pressure  to- 
morrow. 

The  importance  of  this 
shape  of  isobars  in  forecasting 
arises  from  the  fact  that,  as 
both  the  anticyclones  are 
usually  stationary,  the  col 
represents,  as  it  were,  a  line 
of  weakness,  along  which  dis- 
turbances will  be  propagated. 
Unfortunately,  though  a 
col  can  be  safely  forecast  in 
general  terms  as  for  unsettled 
weather  without  much  wind, 
the  motion  of  cyclones  as 
they  meet  a  col  is  most  uncertain.  Sometimes  they 
pass  in  a  south-eastern  direction  across  Europe  between 
the  two  anticyclones,  while  more  frequently  the  main 
body  of  the  cyclone  is  deflected  or  dies  out,  while  an 
irregular  secondary  pushes  its  way  more  or  less  across 
the  col. 


FIG.  24. — Wind  in  a  "  col." 


OEIGIN  OF  ISOBAES. 

Secondary  cyclones,  wedges,  and  straight  isobars  have 
already  been  sufficiently  described,  so  that  we  may  now 


ISOBAES.  149 

conclude  with  a  few  general  remarks  on  the  whole  question 
of  the  shapes  or  forms  of  isobars.  In  the  first  place,  what 
is  the  meaning  of  the  seven  fundamental  shapes  ?  From 
the  analogy  of  water,  to  which  we  have  so  often  referred, 
there  can  scarcely  be  any  doubt  that  just  as  circulating 
water  can  only  take  a  certain  limited  number  of  forms, 
eddies,  backwaters,  ripples,  etc.,  so  air  can  only  move  in 
a  limited  number  of  ways,  of  which  these  seven  are  the 
most  important. 

A  cyclone  may  be  anything  from  two  thousand  to 
fifty  miles  across ;  a  wedge  may  fill  the  whole  North 
Atlantic,  or  it  may  also  be  measured  by  single  miles,  but 
their  respective  characters  are  in  no  way  changed.  Then, 
though  we  are  only  at  present  able  to  give  the  general 
nature  of  the  upper  and  lower  circulation  in  cyclones  and 
anticyclones,  there  is  no  doubt  that  the  other  five  shapes 
are  capable  of  being  worked  out  in  a  similar  manner. 
The  conclusion  is  irresistible.  Just  as  we  proved  that  not 
only  the  broad  features  of  the  weather,  but  also  the 
minute  characteristics  of  every  cloud  in  cyclones  and 
anticyclones  are  the  product  of  the  nature  of  their  respec- 
tive circulations,  so  must  the  weather  and  cloud  in  the 
remaining  five  shapes  be  also  the  product  of  some  form 
of  atmospheric  motion. 

In  an  eddying  river  the  general  cause  of  all  motion  is 
the  downward  current  from  the  source  to  the  mouth ;  but 
water  cannot  slide  like  a  weight  on  an  inclined  plane, 
without  forming  horizontal  or  vertical  whirls.  In  the 
atmosphere  the  prime  source  of  all  motion  is  the  general 
circulation  of  air  set  up  between  the  hot  equator  and  the 
cold  poles.  This,  like  water,  must  form  vortices,  or 


150  WEATHER. 

ripples,  and  isobars  map  out  the  varying  pressure  on  the 
earth's  surface  induced  by  the  uneven  flow  of  air.  If  we 
had  a  series  of  barometers  at  the  bottom  of  a  river,  and 
observed  them  all  simultaneously,  we  should  find  a  defect 
of  weight  under  the  eddies,  with  an  excess  of  pressure 
under  the  backwater,  and  we  might  draw  isobaric  lines 
which  would  map  out  the  position  of  these  vortices 
exactly  as  we  do  in  air. 

But  it  must  be  specially  noted  that  the  depression  of 
a  cyclonic  eddy  is  not  entirely  due  to  centrifugal  force 
like  that  of  a  water  eddy.  The  depression  of  an  ordinary 
cyclone  could  not  be  produced  by  the  centrifugal  force  of 
the  fiercest  hurricane  that  ever  blew ;  and  the  cause  of 
the  diminution  of  pressure  in  a  cyclone  is  at  present 
unknown. 

When  we  discuss  the  changes  shown  on  two  different 
synoptic  charts,  we  shall  talk  of  changes  in  the  shapes  and 
positions  of  isobars  as  if  those  lines  were  graphical  ab- 
stractions. What  we  really  do  is  to  trace  by  their  means 
the  ever-changing  eddies  of  the  atmosphere — the  death 
of  old  ones,  the  birth  of  new  ones,  or  the  fusion  of  several 
existing  systems  into  a  new  disturbance  of  a  different  type. 

To  avoid  all  danger  of  theoretical  errors,  we  simply 
define  the  kinds  of  eddies  by  certain  abstract  shapes  of 
isobars,  and  then,  entirely  from  observation,  we  collate 
certain  definite  kinds  of  weather  and  sky,  with  different 
portions  of  each  isobaric  configuration. 


CHAPTER  V. 

BAEOGEAMS,  THEEMOGBAMS,  METEOGEAMS. 

WE  have  already  seen  that  a  series  of  synoptic  charts 
are,  as  it  were,  a  series  of  bird's-eye  views,  not  only  of  the 
appearance  of  the  sky,  but  also  of  instrumental  readings 
over  a  considerable  area.  Owing  to  the  expense  of 
telegraphy,  these  can  rarely  be  taken  oftener  than  every 
eight  hours,  but  we  all  know  by  experience  that  very 
great  changes  of  weather  may  occur  within  that  time. 
Every  well-equipped  observatory  is  therefore  supplied 
with  instruments  which  record  automatically  the  height 
of  the  barometer,  of  the  thermometer,  the  direction  and 
velocity  of  the  wind,  the  occurrence  and  quantity  of 
rain,  while  a  wet-bulb  thermometer  for  moisture  and  an 
electrometer  are  sometimes  added. 

The  trace  marked  on  paper  by  a  barograph  is  called  a 
barogram ;  that  by  a  thermograph,  a  thermogram ;  both 
or  either  of  the  records  left  by  the  wind-instruments  are 
called  anemograms;  and  if  all  or  several  of  these  are 
combined  in  one  diagram,  the  whole  is  called  a  meteogram, 
because  it  is  a  writing  of  meteorological  instruments. 
Fig.  25  is  a  copy  of  one  of  the  meteograms  which  are 


152 


WEATHEK. 


published  by  the  British  Meteorological  Office,  and  we 
propose  to  devote  several  paragraphs  to  its  consideration, 
so  as  to  explain  how,  by  means  of  these  continuous  records, 
we  can  fill  up  the  gaps  in  the  history  of  weather-changes 


JO.O 


29.0 —  4.0 


28.0- 


W- 
S  - 
E  - 


7  Dec. ,  1874. 

i     i    I     i     i     i    |     i 


i  —  i 


i  —  r 


-20 
JO 


Raiv. 


Direct. 


. 
-E 

-N 


Noon .  Noon . 

FIG.  25. — A  meteogram. 


Noon . 


which  are  shown  at  considerable  intervals  only,  on  synoptic 
charts. 

It  should  be  remarked  that  in  treating  of  these  we 
no  longer  deal  with  generalities,  but  with  the  actual 
changes  and  variations  of  each  day.  Also  that  the  aim 
and  object  of  all  meteorology  is  to  explain  the  details  of 
the  sequence  of  weather  as  it  occurs  at  any  one  place,  if 
possible  with  great  minuteness,  and  that  synoptic  charts 
are  only  a  means  to  that  end.  Ever  since  the  discovery 
of  the  barometer  and  thermometer,  innumerable  attempts 
have  been  made  to  discover  the  nature  of  weather-changes 
by  calculations,  which  were  based  on  the  fluctuations  of 
these  instruments  as  observed  at  any  one  place,  but  with 


fftiNr 
\,ou 

BAROGRAMS,  THERMOGRAMS,  METEOGRAMS.  153 

Tery  moderate  success.  We  shall  very  soon  see  why  this 
was  so;  and,  in  fact,  why  it  could  not  be  otherwise,  from 
the  nature  of  things. 


METEOGRAMS. 

Now  for  our  actual  meteogram  in  Fig.  25.  The 
barometer-trace  is  marked  "bar,"  with  an  appropriate 
scale  at  the  edge.  Temperature  is  marked  "  therm ; " 
while  the  upper  curve  of  all,  marked  "  vel,"  gives  the 
varying  velocity  of  the  wind,  only  we  must  note  that  for 
convenience  its  base  is  taken  from  the  top  of  the  diagram, 
and  that,  therefore,  it  is  measured  downwards.  Eain  is 
marked  near  the  bottom  of  the  diagram  by  oblique  lines, 
which  are  proportional  to  the  amount  of  rain  measured  at 
the  end  of  each  hour,  so  that  we  can  see  at  a  glance  when 
rain  fell  and  how  much  of  it.  Below  all  is  the  trace 
marked  "  direct,"  which  refers  to  the  direction  of  the 
wind.  The  notation  requires  a  word  of  explanation,  as  we 
have  to  represent  an  angular  scale  on  a  rectangular 
diagram.  It  will  be  seen  that  the  bottom  of  the  diagram 
is  marked  N  for  north ;  then  as  we  rise  upwards  we  get 
successively  east,  south,  west,  and  back  to  north  again  at 
the  top  of  the  direction-figure.  By  this  means  we  can  see 
at  a  glance  whether  the  wind  is  veering  or  backing. 
When  the  wind  veers,  the  trace  moves  upwards ;  while, 
when  the  wind  backs,  the  curve  descends.  A  little 
practice  is  necessary  to  read  this  easily,  but  when  once 
learnt  it  is  very  convenient. 

But  to  understand  this  meteogram  fully,  we  must  look 
at  the  charts  given  in  Figs.  26  and  27,  for  the  8th  and  9th 


154 


WEATHER. 


of  December,  1874,  respectively.  The  broad  features  are 
very  simple.  The  cyclone  which  we  see  approaching  in 
the  first  chart  passes  across  England  to  Holland,  where 
we  find  it  in  the  second  chart ;  and  now  let  us  see  how 


9-12-74 
8. a.m. 


FIGS.  26  and  27. — Charts  to  illustrate  meteograrn. 

this  affected  the  sequence  of  weather  at  Stonyhurst,  near 
Manchester,  marked  s  on  the  charts. 

The  meteogram,  Fig.  25,  refers  to  the  three  days, 
December  7  to  9,  1374,  while  the  charts  refer  to  the 
two  latter  days  only.  The  barometer  rose  till  nearly  mid- 
night on  the  7th,  under  the  action  of  the  wedge  which  we 
find  over  the  North  Sea  on  the  chart  for  the  morning  of 
the  8th.  Then  we  see  by  the  trace  that  the  mercury  was 
falling  fast,  owing  to  the  approach  of  a  cyclone.  This  fall 
continued  till  about  midnight,  when  a  rapid  rise  com- 


BAROGRAMS,   THERMOGRAMS,   METEOGRAMS.  155 

menced,  which  lasted  all  the  next  day.  The  charts  alone 
would  scarcely  enable  us  to  mark  the  exact  line  of  the 
passage  of  the  cyclone,  but  from  the  wind-traces  we  know 
that  the  centre  passed  very  nearly  exactly  over  Stony- 
hurst. 

The  temperature-curve  is  much  more  complicated. 
On  the  first  day  we  see  a  most  irregular  curve,  with  little 
trace  of  the  ordinary  diurnal  range  of  temperature,  for 
the  highest  and  lowest  points  were  reached  within  about 
an  hour  of  each  other,  and  in  the  middle  of  the  day.  This 
can  be  easily  explained.  The  station  was  the  whole 
day  under  the  influence  of  the  rear  of  a  cyclone,  with  a 
north-west  wind  and  cold  showers.  These  and  driving 
clouds  gave  rise  to  the  sudden  changes  which  are  marked 
on  the  curve.  Next  day  there  are  more  signs  of  diurnal 
variation,  but  the  highest  temperature  was  at  6  p.m.,  and 
the  second  midnight  is  much  hotter  than  the  first.  Also, 
though  rain  was  collected  almost  every  hour,  we  do 
not  see  the  sudden  changes  of  the  preceding  day.  The 
interpretation  of  all  this  is  as  follows  : — 

Commencing  from  the  early  morning,  the  wind  began 
to  back  from  west-north-west  to  south  and  south-south- 
east. This  increased  the  general  warmth  at  the  second 
midnight,  and  the  diurnal  range  took  its  usual  course  on 
the  top  of  the  more  general  change.  Why  the  rain  did 
not  send  down  the  thermometer  will  be  obvious  when  we 
know  that  this  all  happened  in  the  front  of  an  intense 
cyclone,  and  we  remember  that  the  soft,  warm,  drizzling 
rain  which  falls  there  is  not  like  the  chilly  showers  of  the 
rear  of  a  depression. 

On  the  third  day  the  thermogram  is  still  more  curious. 


156  WEATJIEE. 

Immediately  after  midnight  the  thermometer  fell  5°  Fahr., 
just  as  the  trough  of  the  cyclone  passed,  and  the  wind 
jumped  from  south-west  to  north.  During  the  whole  day 
the  temperature  fell,  and  while  the  greatest  heat  was  at 
the  first,  the  greatest  cold  was  at  the  second,  midnight. 
No  rain  was  reported,  so  the  small  sudden  changes  must 
be  the  effect  of  passing  clouds.  Here  it  may  be  well 
to  note  that  a  passing  cloud  by  day  will  send  down  the 
thermometer  by  hiding  the  sun,  while  by  night  the  same 
cloud  will  raise  temperature  by  cutting  off  the  radiation 
of  the  cold  sky.  The  general  explanation  is  that  cold 
winds  set  in  when  the  barometer  turned,  and  that  their 
influence  apparently  completely  overrode  diurnal  varia- 
tions. 

Though  the  thermometer  fell  all  day — that  is  to  say, 
there  was  no  hour  at  which  the  temperature  was  not  lower 
than  at  the  preceding  one — it  does  not  follow  that  there 
was  no  diurnal  range  with  a  regular  maximum  and 
minimum ;  but,  as  the  explanation  of  the  superposition 
of  curves  requires  some  collateral  details,  we  will  defer 
our  remarks  till  we  have  considered  the  wind-traces  of 
our  meteogram. 

First,  then,  for  the  velocity-trace.  Presuming  that 
the  normal  course  of  the  wind  is  to  increase  regularly  in 
velocity  from  4  a.m.  till  2  p.m.,  and  then  fall  gradually 
again,  the  most  obvious  feature  in  our  wind-diagram  is 
the  increase  of  the  wind  towards  the  middle  of  the  day ; 
that  is  to  say,  the  regularity  of  this  diurnal  variation  in 
spite  of  the  great  cyclonic  changes  which  were  going  on. 
The  influence  of  these  latter  is  seen  in  the  comparatively 
high  velocities — from  thirty  to  forty  miles  an  hour — which 


BAROGRAMS,   THERMOGRAMS,   METEOGRAMS.  157 

the  wind  attained  a  little  after  noon  on  the  first  two  days  ; 
but  the  trace  for  the  last  day,  December  9,  is  very  different. 
On  that  day  the  strongest  winds  were  before  6  a.m.,  and 
during  the  hottest  time  of  the  day,  when  the  wind  is 
usually  strongest,  the  velocities  decreased  steadily.  The 
reason  of  all  this  was  that,  in  the  early  morning,  the  steep 
gradients  in  rear  of  the  cyclone  were  passing  over  the 
station,  while  in  the  middle  of  the  day  the  gradients  were 
becoming  so  much  less  steep  that  the  diminishing 
velocity  due  to  them  entirely  overrode  any  increase  which 
would  naturally  have  occurred  from  diurnal  influences. 
The  calm  just  before  midnight  on  the  8th,  and  the  rapid 
rise  of  the  wind  to  thirty  miles  an  hour  directly  afterwards, 
are  very  interesting  ;  for  the  calm  is  that  in  the  centre  of 
an  intense  cyclone,  and  the  high  wind  is  associated  with 
the  steep  gradients  which  we  see  in  rear  of  the  depression 
in  Fig.  27. 

The  direction-changes  for  these  three  days  are  tolerably 
simple.  The  natural  diurnal  variation  of  the  wind  is  to 
veer  a  little  in  the  forenoon,  and  back  again  as  the  sun 
goes  down ;  but  almost  all  trace  of  this  is  lost  in  the  stormy 
weather  to  which  our  diagram  refers.  On  the  7th  the 
wind  kept  pretty  steady  between  west  and  north-west, 
and  there  is  little  sign  of  diurnal  variation.  Next  day, 
the  8th,  as  the  cyclone  approached,  the  wind  backed 
rapidly  as  far  as  south-south-east ;  then  veered  rapidly  to 
south-west ;  and  just  at  midnight,  when  the  barometer 
turned,  jumped  up  to  the  north  and  north-east ;  and  then 
backed  during  the  day  to  about  north-west.  From  these 
changes,  and  the  sudden  jump  of  the  wind  from  west  to 
north,  it  is  evident  that  the  cyclone's  centre  passed  very 


158  WEATHEE. 

nearly  over  the  station.     All  traces  of  diurnal  variation 
are,  of  course,  entirely  marked  by  these  greater  changes. 

SUPEKIMPOSITION   OF   VARIATIONS   ON   CURVES. 

The  simple  conception  that  the  actual  weather  is  the 
balance  or  sum  of  various  influences  will  now  be  sufficiently 
obvious,  but  we  must  go  a  little  more  into  detail  of  all 
that  can  be  learnt  from  an  inspection  of  instrumental 
curves.  As  the  idea  of  superimposing  curves  of  different 
kinds  of  variation  one  on  the  top  of  another,  and  of  so 
deducing  a  resulting  curve,  may  not  be  familiar  to  some 
of  our  readers,  we  will  commence  by  an  easy  example. 

Here,  and  all  through  this  work,  we  shall  use  con- 
ventionally the  word  "  changes  "  to  denote  alterations  in 
weather  due  to  cyclones,  etc. ;  and  "  variations  "  to  denote 
alterations  due  to  the  time  of  day.  The  passage  of  a 
cyclone,  or  its  replacement  by  an  anticyclone,  really 
changes  the  weather;  diurnal  influences  only  impose  a 
certain  variation  on  these  greater  changes. 

For  instance,  let  us  try  and  find  out  what  would  be 
the  nature  of  the  curve  left  by  a  thermograph  if  a  regular 
diurnal  variation  of  temperature,  which  was  highest  at 
2  p.m.  and  lowest  at  4  p.m.,  was  superimposed  on  a 
steady  general  fall  of  temperature,  due  to  other  than 
diurnal  influences — say,  the  setting  in  of  cold  northerly 
winds.  The  line  on  which  the  diurnal  curve  is  super- 
imposed is  called  the  level  of  variation.  In  the  familiar 
curve  of  mean  diurnal  variation,  such  as  B  or  c  in 
Fig.  28,  the  straight,  horizontal  line  A  represents  the 
mean  temperature  of  the  place,  and  the  curves  B  or  c 


BAROGRAMS,   THERMOGRAMS,  METEOGRAMS.  159 


are  the  resulting  traces.  If  they  are  unaltered,  the  only 
effect  of  any  change  in  the  level  of  A  would  be  to  bring 
the  whole  nearer 
or  further  from 
the  base  of  the 
figure.  For  the 
level  of  A  might 
either  be  40°  or 
70°,  but  the 
shape  and  mag- 
nitudes of  the 
diurnal  curves  B 
or  c  would  be 
the  same. 

When  we  come 
to  deal  with  the 
significance  of  a 
curve  for  any 
particular  day, 
the  horizontal 
line  A  no  longer 
represents  the 
mean  tempera- 
ture of  the  sta- 
tion, but  the 
level  of  tempera- 
ture from  gene- 
ral causes  inde- 
pendent of  the 
time  of  day.  If 

the  thermograph  FIG.  28.— Superlinposition  of  curves. 


150  WEATHER. 

gave  such  a  trace  as  B  or  c,  it  is  possible,  from  geometrical 
considerations,  to  draw  the  line  A,  which  denotes  the  level 
of  general  temperature  on  which  the  diurnal  curves  are 
superimposed.  The  method  is  as  follows.  As  B  and  c 
are  both  diurnal  curves,  which  only  differ  in  magnitude, 
but  not  in  character,  we  will  confine  our  attention  to  c 
only.  Then,  if  the  instrumental  trace  gave  c,  find  from 
it  the  mean  temperature  of  the  day  by  taking  the 
mean  of  the  readings  at  every  hour ;  mark  this  value  m 
on  the  vertical  hour-line  for  noon;  join  n  and  t,  the 
points  where  the  trace  cuts  the  first  and  second  mid- 
night hour-lines ;  then  a  line  A  drawn  through  m  parallel 
to  nt  is  the  level  of  variation  required.  The  line  nt  is 
omitted  in  the  diagram  for  the  sake  of  greater  clear- 
ness. 

But  now  suppose  that  the  diurnal  variation  B  or  c 
remained  the  same,  but  that  from  other  causes  the  general 
temperature  fell  so  uniformly  that  it  may  be  represented 
by  a  straight  line,  such  as  D,  what  would  the  resulting 
trace  be  like  ?  This  we  can  easily  find  graphically,  by 
drawing  the  line  D  and  taking  it  as  the  variable  level  on 
which  to  add  B  and  c.  To  do  this  we  have  only  to 
measure  the  values  of  B  and  c  at  different  hours  from 
the  level  of  the  line  D  at  the  same  hour.  For  instance, 
the  minima  at  4  a.m.  of  E  and  F  are  the  same  distance 
from  D  that  B  and  c  are  from  A  at  the  same  hour ;  also 
at  8  a.m.  and  8  p.m.  the  diurnal  curves  equally  cross  the 
line  of  general  level,  and  the  maxima  at  2  p.m.  are  also 
at  equal  distances  from  that  level.  Now  look  at  the 
resulting  curves  E  and  F.  The  latter,  which  is  the 
stronger,  is  able,  as  it  were,  to  reverse  the  general  fall  of 


BAROGRAMS,   THERMOGRAMS,   METEOGRAMS.  161 

the  line  D,  and  every  one  would  recognize  that  there  was 
a  diurnal  range.  But  then  turn  to  curve  E.  Here  the 
diurnal  variation  is  so  small  that  it  can  only  deflect,  but 
not  reverse,  the  general  line  D,  and  thus  we  get  the 
apparently  impossible  result  that  the  thermometer  may 
fall  all  day,  and  yet  that  there  may  be  a  very  distinct 
diurnal  maximum  and  minimum,  which  only  modify  the 
rate  of  fall  of  the  general  sweep  of  the  curve.  Now,  this 
is  exactly  what  happened  on  December  9,  1874,  at  Stony- 
hurst,  as  shown  in  our  meteogram  (Fig.  28).  There  the 
thermometer  fell  all  day,  but  by  joining  mentally  the 
points  where  the  trace  cuts  the  first  and  second  midnights 
of  that  day,  we  see  at  once  that  there  is  a  diurnal  maximum 
about  3  p.m.  in  the  general  sweep  of  the  curve.  In  this 
case  the  curve  is  so  irregular  that,  though  we  can  detect 
the  fact  of  a  diurnal  maximum,  we  cannot  measure  the 
amount  even  approximately.  When,  however,  the  trace 
is  more  regular,  it  is  obvious  that  from  a  curve  like  E  we 
could  infer  D,  and  the  maximum  and  minimum  diurnal 
values  of  E  by  a  method  exactly  similar  to  that  which  we 
employed  to  find  A  from  B.  In  fact,  given  B,  c,  and  D, 
we  can  draw  E  and  F ;  while,  given  E  and  F,  we  can 
separate  them  into  a  general  line  D  and  diurnal  variations 
B  and  c  respectively. 

If  the  line  of  general  level  is  so  irregular  that  it 
cannot  be  represented  by  a  straight  line  for  twenty-four 
consecutive  hours,  then  we  can  no  longer  separate  the 
general  and  diurnal  changes,  for  we  are  unable  to  draw 
the  general  level,  which  is  then  a  curved  line. 

We  have  taken  our  illustration  from  temperature- 
curves,  as  their  diurnal  changes  are  the  simplest  and  most 

M 


162  WEATHER. 

obvious ;  but  the  same  general  principles  apply  to  every 
meteorological  element. 

In  barograms  the  diurnal  variations  in  Great  Britain 
are  so  small  compared  to  the  general  changes  that  the 
former  can  usually  be  neglected ;  but  in  the  tropics  the 
mercury  often  falls  regularly  O12  in.  in  six  hours,  while 
the  general  changes  of  an  approaching  distant  hurricane 
reach  half  that  amount.  Then  the  discrimination  between 
general  changes  and  diurnal  variation  of  pressure  is  of  vital 
importance. 

The  question  of  sorting  out  various  sources  of 
barometric  movement  is  so  important  in  every  branch  of 
meteorology  that  we  must  give  the  subject  in  greater 
detail. 


BAROMETRIC  KATE. 

The  measure  of  the  rapidity  with  which  the  mercury 
rises  or  falls  is  called  the  "  barometric  rate."  This  is 
usually  expressed  by  saying  how  many  hundredths  of  an 
inch,  or  tenths  of  a  millimetre,  the  mercury  changes  in  an 
hour. 

In  a  variable  climate  like  Great  Britain,  anything 
under  0*02  in.  per  hour  is  a  low  rate,  while  anything 
over  O05  may  be  considered  high.  On  only  a  few  occa- 
sions in  any  year  will  O10  be  exceeded,  though  as  high  as 
O20  has  been  recorded  in  exceptional  cases.  In  the 
tropics  diurnal  variation  alone  may  give  a  rate  of  nearly 
O02,  and  anything  higher  than  this  would  be  a  warning 
of  danger. 

As  many  people  have  a  very  vague  and  inaccurate 


BAROGRAMS,   THERMOGRAMS,   METEOGRAMS.  163 

idea  of  the  relation  of  cyclone-motion  and  gradients  to 
barometric  rate,  it  may  be  expedient  to  give  some 
further  explanations.  If  we  look  at  the  diagram  of  a 
cyclone  given  in  Fig.  29,  it  is  obvious  that  the  rate  of  fall 
of  A'S  barometer  will  depend  on 
three  things — the  velocity  of  the 
cyclone ;  the  steepness  of  the 
gradients ;  and  the  position  of  the 
observer  relative  to  the  cyclone- 
centre.  If  we  assume  that  a  dis- 
tance equal  to  A  B  would  be 
traversed  by  the  cyclone-centre 
in  four  hours,  A'S  rate  would  be 

IUL  =  0*025,  as  the  distance  be-  FIG.   29.— Barograms,    baro. 
,1-1  •      A  i    •  i          metric  rate,  and  filling  up 

tween  the  isobars  is  O'l  in. ;  and       of  cyciones. 
it  is  evident  that  this  rate  might 

be  doubled  by  doubling  either  the  velocity  of  the  cyclone 
or  the  barometric  difference  between  the  isobars.  Then, 
in  the  same  cyclone,  with  the  same  velocity  and  gradients, 
the  rate  to  E,  who  was  in  the  line  of  the  cyclone's  path, 
would  be  much  greater  than  that  to  A,  who  was  more 
remote.  This  is  manifest,  because,  as  a  cyclone  is  generally 
nearly  round,  the  shortest  line  between  two  concentric 
isobars  is  along  a  radius  of  the  circle.  In  this  diagram 
the  distance  E  D  is  ab  >ut  half  A  B,  so  that  E'S  barometric 
rate  would  be  double  that  of  A. 

From  these  considerations  we  can  see  that. a  rapid  fall 
of  the  barometer  is  dangerous,  because,  in  a  general  way, 
it  shows  that  the  observer  is  nearly  in  a  line  with  the 
path  of  the  cyclone,  that  the  gradients  are  steep,  and  that 
the  disturbance  is  moving  rapidly.  The  first  gives  an 


104  WEATHER. 

almost  complete  reversal  of  the  wind,  which  is  most 
dangerous  to  ships ;  the  second,  high  wind  ;  while  the 
third  increases  the  intensity  of  the  weather  in  every 
way. 

When  we  come  to  discuss  squalls  and  thunderstorms 
we  shall  find  that  the  barometer  often  jumps  up  O'l  in. 
in  a  few  minutes,  just  as  the  heavy  rain  begins.  The 
cause  of  this  is  uncertain,  but  we  must  notice  that  the  rise 
is  of  a  totally  distinct  nature  from  that  produced  by  the 
passage  of  a  cyclone.  We  must  not,  therefore,  be  led 
into  the  error  of  treating  all  barometric  changes  as 
identical,  and  of  comparing  the  barometric  rate  of  a  squall 
with  that  of  a  cyclone.  A  difficult  case  arises  sometimes 
with  the  squall  in  the  trough  of  a  cyclone.  As  the  wind 
goes  round  with  gusts  and  he  ivy  rain,  the  mercury  turns 
upwards  a  id  rises  with  a  sudden  jump.  Sometimes  a 
slight  fall  then  occurs,  but  directly  afterwards  the  baro- 
meter rises  quickly  and  steadily.  It  is  almost  impossible 
to  say  how  much  of  the  first  jump  is  due  to  the  squall, 
and  how  much  to  the  general  increase  of  pressure  due  to 
the  passing  on  of  the  cyclone. 

We  have  already  explained  that  a  barogram  is  a 
section,  as  it  were,  of  a  cyclone  which  is  seen  in  plan  on 
a  synoptic  chart.  This,  however,  evidently  only  holds 
good  on  the  supposition  that  the  cyclone  changes  neither 
in  depth  nor  shape  during  the  time  whicli  elapses  between 
the  beginning  and  the  end  of  the  trace.  Now,  in  practice 
a  cyclone  is  perpetually  changing  both  its  depth  and 
shape;  and,  consequently,  it  is  often  extremely  difficult 
to  see  how  the  changes  of  pressure  which  are  seen  in  two 
synoptic  charts  at  even  a  short  interval  would  have 


BAROGRAMS,  THERMOGRAMS,  METEOGRAMS.     165 

influenced  the  trace  of  the  barometer  at  any  one  place. 
Two  or  more  sets  of  general  changes  are  going  on  simul- 
taneously, and  we  have  to  work  out  the  result  of  their 
combination.  But  the  importance  of  investigating  the 
question  will  be  evident  when  we  remark  that  on  this 
depends  all  the  apparently  anomalous  movements  of  the 
barometer.  If  the  mercury  always  fell  before  rain,  and 
rose  when  the  weather  began  to  mend,  meteorology  would 
be  the  simplest  of  sciences  ;  but,  unfortunately,  we  often 
see  rain  while  the  barometer  is  rising,  or  the  sky  begin 
to  clear  while  pressure  is  still  on  the  decrease.  These 
exceptions  have  not  been  hitherto  explained,  but  in  this 
work  we  propose  to  do  so. 

Suppose  that,  as  in  Fig.  29,  a  cyclone,  with  centre  at 
C,  was  moving  along  the  crossed  lane  marked  "  path,"  at 
such  a  rate  that  it  would  traverse  a  distance  equal  to 
A  B  in  four  hours.  .Then — noting  that  the  isobars  at  A 
and  B  are  Ol  inch  of  pressure  apart ;  that  the  line  A  B 
is  parallel  to  the  path  of  the  cyclone ;  and  that  B  is  on 
the  line  of  the  trough,  where  the  barometer  naturally 
begins  to  rise — if  the  cyclone  moved  onwards  without 
any  change  in  the  depth  of  the  centre,  which  is  29'0  in., 
the  barometer  at  station  A  would  fall  0*10  in.  in  these  four 
hours,  and  then  commence  to  mount. 

But  now,  suppose  that,  while  the  centre  moved 
onwards,  the  cyclone  began  to  fill  up  at  the  rate  of  O'lb* 
in.  in  the  four  hours  (and  this  is  quite  within  practical 
limits),  then  the  barometer  at  A  would  rise  on  balance, 
0*06  in.  in  that  time.  In  fact,  it  niight  be  supposed  to 
fall  (HO  in.  from  the  approach  of  the  cyclone's  trough 
but  to  rise  0'16  in.  from  filling  up,  so  that  a  gain  of 


166  WEATHER. 

0*0(5  in.  would  remain.  Thus  we  explain  the  apparent 
anomaly  of  the  barometer  rising  while  a  cyclone  ap- 
proaches ;  and  here  we  see  the  enormous  gain  to  know- 
ledge which  synoptic  charts  have  effected.  Formerly 
these  barometric  anomalies  were  inexplicable ;  now  we 
can  interpret  them  readily,  for  we  know  that  rain  and 
wind  depend  on  the  shape,  and  not  the  level,  of  the  isobars. 
So  that,  though  the  cyclone  is  filling  up  and  the  barometer 
rising,  the  wind  and  weather  at  station  A  remain  charac- 
teristic of  the  front  of  a  cyclone.  We  shall  refer  to  this 
subject  aoain,  and  give  a  striking  example  in  our  chapter 
on  Forecasting  for  Solitary  Observers. 

SURGE. 

Any  change  of  barometric  level  which  is  not  due  to 
the  passage  of  some  sort  of  depression  or  diurnal  varia- 
tion is  called  a  "  surge  "  of  pressure.  The  word  "  wave  " 
has  often  been  applied  to  barometric  changes,  but  in  such 
an  uncertain  way  that  it  seems  best  to  coin  a  new  word 
for  a  very  definite  and  important  phenomenon. 

We  have  just  explained  the  idea  of  a  moving  cyclone 
filling  up,  and  of  the  resulting  balance  of  a  gain  of  pressure. 
It  would  have  been  just  as  easy  for  the  cyclone  to  grow 
deeper  in  the  same  time,  when  we  should  have  had  the 
barometer  falling  in  rear  of  the  cyclone,  with  clearing 
weather.  Sometimes  filling  up  of  a  cyclone  is  tolerably 
local ;  other  times  surging  is  on  an  enormous  scale. 
Nothing  is  more  commou  in  winter  than  to  find  a 
moderate-sized  cyclone  in  mid-Atlantic  one  day,  and 
that,  though  by  next  morning  the  shape  of  the  isobars  has 


BAROGRAMS,  THERMOGRAMS,  METEOGRAMS.  167 

hardly  changed,  the  whole  level  of  the  cyclone  and  sur- 
roundings has  perhaps  decreased  half  an  inch. 

This  hardly  shows  at  first  on  a  synoptic  chart,  for  you 
see  no  change  in  the  configuration  of  the  lines ;  but,  on 
looking  at  the  figures  attached  to  the  isobars  which 
denote  the  level,  you  see  that  what  was  29*5  in.  the  first 
day  is  only  29*0  in.  on  the  following  morning.  In  like 
manner,  a  persistent  anticyclone  will  often  rise  and  fall  one 
or  two-tenths  of  an  inch  without  any  motion  or  material 
change  of  shape  on  the  chart,  while  the  barometer  at  any 
station  will  have  appeared  to  rise  or  fall  without  any 
reason  or  apparent  change  of  weather. 

When  we  look  at  a  series  of  these  surges  we  find  a 
decided  tendency  of  the  motion  to  travel  from  west  to 
east,  or  from  south-west  to  north-east.  For  instance, 
suppose  that  one  day  there  was  a  deep  depression  with 
one  or  more  cyclones  in  the  United  States,  an  anti- 
cyclone in  mid- Atlantic,  and  a  shallow  set  of  depressions 
over  Europe.  We  might  find  by  next  morning  that  the 
American  cyclones  were  filling  up,  but  that  the  Atlantic 
high  pressure  was  lower  in  level,  but  unchanged  in  posi- 
tion, while  the  European  system  was  practically  un- 
altered. The  third  day  might  see  that,  with  little  change 
in  America,  the  Atlantic  anticyclone  had  regained  its 
former  level,  while  a  great  decrease  of  pressure  had 
occurred  over  the  whole  of  Europe. 

This  is  the  "barometric  wave"  of  Birt  and  other 
writers,  to  which  little  importance  is  now  attached,  but 
which,  the  author  believes,  contains  the  germ  of  great 
development.  The  insuperable  difficulty  of  tracing  waves 
at  present  arises  from  the  impossibility  in  most  cases  of 


168  WEATHER. 

separating  the  total  barometer-change  into  its  two  com- 
ponents of  surge  and  cyclone.  Nothing  is  easier  than  to 
record  the  hour  at  which  any  barometer  has  touched  its 
lowest  point,  but  we  cannot  tell  how  much  is  due  to  the 
depression  of  a  cyclone,  or  to  the  depression  of  a  surge. 
It  is  manifest,  from  our  last  diagram,  that  we  only  observe 
when  the  balance  is  lowest. 

A  surge  of  itself  has  no  characteristic  weather,  but  the 
passage  of  a  surge  exercises  a  moderate  influence  on  the 
characteristic  weather  of  any  isobaric  shape,  and  a  very 
powerful  one  on  the  formation  of  new  systems. 

Let  us  define  the  front  of  a  surge  as  all  the  part  where 
pressure  is  decreasing  ;  the  rear  as  all  the  part  where 
pressure  is  increasing ;  the  trough  as  the  line  of  change 
from  fall  to  rise  ;  and  the  crest  as  the  line  of  change  from 
rise  to  fall.  Then  we  find  that  the  front  of  a  moving 
surge,  or  the  mere  deepening  of  a  cyclone,  does  not  alter 
the  typical  character  of  the  front  and  rear  of  the  cyclone, 
but  increases  the  general  intensity  ;  while  the  rising  part 
of  a  surge  decreases  the  intensity,  and  so  improves  the 
weather.  The  lowering  of  an  anticyclone  decreases  the 
dryness  and  increases  the  tendency  to  form  cloud,  while 
gain  of  pressure  has  the  opposite  effect.  But  the  most 
striking  and  by  far  the  most  important  effect  of  surge  is 
the  influence  on  the  development  of  new  systems  of  dis- 
turbance. The  tendency  of  all  reduction  of  barometric 
level  all  over  the  world  is  to  induce  cyclonic  systems, 
while  that  of  gain  of  pressure  is  to  dissipate  existing 
cyclones. 

We  shall  find  abundant  examples  of  this  great  prin- 
ciple in  our  illustrations  of  types  of  temperate  weather. 


BAROGRAMS,   THERMOGRAMS,   METEOGRAMS.  169 

There,  as  we  have  just  mentioned,  surge  and  cyclone  are 
so  mixed  up  together  that  we  can  only  partially  dis- 
entangle them  ;  but  in  the  tropics  we  find  the  same  law 
under  simpler  conditions.  For  instance,  in  the  South 
Indian  Ocean,  during  the  period  of  the  north-west  monsoon 
— from  about  December  to  March — there  is  a  long  furrow 
of  low  pressure  about  10°  south  latitude,  where  the  north- 
west monsoon  meets  the  south-east  trade.  During  the 
whole  of  that  season  this  general  depression  goes  through 
a  series  of  small  surges,  gradually  lowering,  perhaps  one- 
tenth  of  an  inch  for  six  or  seven  days,  and  then  rising 
about  the  same  amount  in  another  week.  Now,  as  a 
matter  of  observation,  hurricanes  almost  invariably  form 
during  the  downward  period  of  the  surge,  and  practical 
forecasters,  like  Meldrum  at  the  Mauritius,  are  always 
specially  on  the  look-out  for  signs  of  serious  bad  weather 
whenever  there  is  the  slightest  symptom  of  a  non-diurnal 
diminution  of  pressure.  We  believe  that  the  same  prin- 
ciple of  watching  surges  might  be  used  for  forecasting 
with  great  advantage  in  temperate  rf  gions,  in  spite  of  the 
difficulties  in  the  way  of  practical  application,  to  which 
we  have  already  alluded. 

It  is  absolutely  necessary,  in  dealing  with  such  com- 
plicated problems  as  occur  in  meteorology,  to  have  short 
simple  terms  to  denote  certain  sets  of  phenomena.  Here, 
and  throughout  this  work,  we  shall  talk  of  all  changes  of 
pressure  due  to  the  passage  of  an  unchanging  area  of  low 
pressure  as  cyclonic-changes,  and  all  due  to  surging  or 
re-arrangement  of  existing  systems  as  surging  changes ; 
and  we  shall  talk  of  surge  overriding  the  cyclone,  or 
cyclone  overriding  the  surge,  when,  in  our  chapter  on 


170  WEATHER. 

Forecasting  for  Solitary  Observers,  we  have  to  explain 
more  fully  many  apparent  anomalies  in  the  behaviour  of 
the  barometer. 


INTERPRETATION  ON  METEOGRAMS. 

We  must  now  examine  still  further  the  relation  of 
charts  to  ineteograms,  and  explain  their  respective  values 
and  interpretations. 

Synoptic  charts  in  practice  can  rarely  be  constructed 
more  than  three  times  a  day,  and  it  is  obvious  that, 
though  general  changes,  such  as  the  formation  or  motion 
of  cyclones,  can  be  shown  on  them  with  the  greatest  clear- 
ness, the  nature  of  diurnal  variations  could  not  be  properly 
discovered  by  their  means.  We  shall  illustrate  this  more 
fully  in  our  chapter  on  Diurnal  Weather,  but  here  we 
must  consider  how  to  collate  .the  variations  due  to  the 
time  of  day  with  the  general  changes. 

Almost  all  over  the  world  the  velocity  of  wind  in- 
creases with  the  day  and  falls  during  the  night,  as  we  saw 
in  our  meteogram,  Fig,  25,  and  this  occurs  both  in 
cyclones  and  anticyclones.  How  cnn  we  collate  this  with 
the  fact  that,  from  charts  constructed  at  the  same  hour  on 
different  days,  the  velocity  is  proportional  to  the  different 
isobaric  gradients  ? '  The  answer  obviously  is,  that  if  we 
suppose  the  gradient  to  remain  unchanged  for  twenty-four 
hours,  the  mean  velocity  of  the  wind  may  be  considered 
the  speed  due  to  that  gradient,  and  that  a  diurnal  varia- 
tion of  velocity  for  gradient  is  superimposed  on  this.  For 
instance,  suppose  the  wind  to  vary  diurnally  between  ten 
and  twenty  miles  per  hour  during  any  day,  so  that  the 


BAROGRAMS,  THERMOGRAMS,   METEOGRAMS.  171 

mean  velocity  was  fifteen  miles,  and  the  variation  due  t» 
diurnal  influences  ten  miles  an  hour ;  also  that  the 
gradient  remained  unchanged :  then  a  synoptic  chart 
constructed  at  the  hour  of  minimum  wind — about  4  a.m. 
— would  give  a  velocity  of  ten  miles  an  hour  for  the 
given  gradient,  while  another  constructed  at  the  hour  of 
maximum  wind — say  at  2  p.m. — would  give  a  velocity  of 
twenty  miles  an  hour  for  the  same  gradient,  and  so  on  fur 
every  other  hour. 

The  diurnal  variation  of  direction  introduces  some 
other  considerations.  In  tbe  temperate  regions  of  the 
northern  hemisphere,  the  wind  usually  veers  a  little 
during  the  day  and  backs  again  at  night,  from  whatever 
direction  it  may  come.  If,  then,  we  consider  the  angle 
between  the  wind  and  the  isobar,  the  above  means  that 
the  angle  is  less  by  day  than  during  the  night.  When 
we  stand  with  our  backs  to  the  wind,  we  are  generally  at 
an  angle  of  about  35°  to  the  left  of  the  isobar ;  so  that 
if  the  wind  veers,  say,  from  south  to  south-south-west 
during  the  day,  while  the  lie  of  the  isobar  remains  the 
dame,  the  angle  between  the  wind  and  isobar  would  bd 
diminished. 

It  has  already  been  noticed  that  the  wind  in  a  cyclone 
is  always  incurved,  while  in  an  anticyclone  it  is  out" 
curved.  We  therefore  infer,  from  the  fact  of  the  mean, 
diurnal  veering  of  wind,  that  in  cyclones  the  wind  is  a 
little  less  incurved,  and  in  anticyclones  a  little  less  out- 
curved,  by  day  than  by  night. 

The  following  will  illustrate  the  above  principles. 
In  Figs.  30,  31,  and  32  we  give  a  reduction  of  the 
United  States  daily  charts  for  January  20,  1873,  at 


172 


WEATHER. 


11  pm.,  together  with  that  at  4.35  p.m.  and  11  p.m.  on 
January  21. 

These  charts  may  be  taken  as  representing  a  freely 
moving  cyclone,  the  intensity  of  which,  as  measured  by 
the  gradients,  is  pretty  constant;  bat  when  we  look  at 
the  wind-arrows  it  will  be  seen  that,  while  in  the  two 
1 1  p.m.  charts  there  is  only  one  station  in  the  first  where 


FIG.  30. — Dinrnal  variation  of  wind  in  a  cyclone. 

the  wind  exceeds  twenty  miles  an  hour,  and  none  in  the 
second,  the  435  p.m.  chart  not  only  has  three  stations 
with  that  velocity,  and  one  over  thirty  miles,  but  contains 
a  far  larger  number  of  arrows  indicating  more  than  ten 
miles  an  hour,  as  shown  by  the  feathers  on  the  arrows. 
The  original  records  show  that  the  total  miles  of  wind  at 
all  the  seventy-five  reporting  stations  in  the  first  chart  is 
449  miles,  with  eight  calms;  i.i  the  second,  681  miles, 


BAROGRAMS,  THERMOQRAMS,   METEOGRAMS.  173 

wilh  only  five  calm  stations;  while  in  the  third  chart,  the 
wind  has  fallen  to  420  miles,  and  the  calms  have  increased 
to  twelve,  though  the  gradients  remain  pretty  constant. 

Next  as  regards  the  diurnal  variation  in  the  wind's 
direction.  Though  not  very  obvious,  still,  on  the  whole 
the  arrows  in  the  4.35  p.m.  chart  will  be  found  rather 
less  incurved  than  in  either  of  those  at  11  p.m.  relative 


FIG.  31. — Diurnal  variation  of  wind  in  a  cyclone. 

to  the  cyclone-centre  ;  so  that  at  every  station  the  wind, 
from  whatever  direction  it  may  blow,  appears  to  veer  a 
little  with  the  sun  during  the  day.  and  to  back  towards 
night,  unless  overridden  by  the  greater  changes  due  to 
the  cyclone's  motion.  Similarly,  if  the  charts  had  been 
constructed  at  the  same  hours  for  an  anticyclone,  the 
wind  would  have  been  found  a  little  less  outcurved  at 
4.35  p.m.,  and  at  every  station  the  wind  would  also  have 


174  WEA.THER. 

veered  a  little  during  the  day,  and  then  backed  towards 
evening. 

The  relation  of  the  weather  in  isobars  to  the  diurnal 
variation  in  the  frequency  of  rain  and  cloud  at  different 
hours  is  rather  more  complicated.  Suppose  that  all  rain 
was  cyclonic,  arid  that  the  curve  of  mean  diurnal  fre- 
quency of  rain  showed  a  maximum  at  2  p.m.,  and  a 


FIG.  32. — Diurnal  variation  of  wind  in  a  cyclone. 

minimum  at  4  a.m.,  what  difference  should  we  expect  to 
see  in  a  synoptic  chart  for  any  particular  cyclone  if  it 
was  constructed  for  those  two  hours  ?  The  inference 
undoubtedly  is  that  the  general  position  of  rain  and 
cloud  relative  to  the  lowest  isobars  would  be  unchanged, 
but  that  the  rain  and  cloud  would  extend  further  from 
the  centre  at  2  p.m.  than  at  4  a.m.  Thus,  if  we  could 
conceive  a  stationary  and  unchanging  cyclone,  it  would 


BAROGRAMS,  THERMOGRAMS,  METEOGRAMS. 


175 


rain  the  whole  twenty-four  hours  to  an  observer  inside 
the  diminished  rain-area  which  the  4  a.m.  chart  would 
show,  while  it  would  begin  to  rain  in  the  morning  and 
cease  towards  evening  to  an  observer  situate  anywhere 
between  this  and  the  outside  of  the  2  p.m.  extension  of 
the  rain-area.  As  an  illustration,  we  give  in  Figs.  33, 
34,  and  35,  in  a  diagrammatic  form,  the  position  of  cloud 


FIG.  33. — Diurnal  variation  of  rain  and  cloud  in  a  cyclone. 

and  rain  in  a  typical  cyclone  of  pretty  constant  shape 
and  gradients  in  the  United  States  on  January  20,  1873, 
at  11  p.m.,  and  on  the  21st  at  4.35  p.m.,  being  the  same 
cyclone  whose  winds  have  already  been  discussed. 

From  these  it  will  be  readily  seen  that  the  area  of 
rain  and  cloud  round  the  centre  in  the  first  11  p.m.  chart 
is  considerably  increased  in  the  4.35  p.m.  chart,  and  again 
diminished  in  size  in  the  second  11  p.m.  chart.  The 


176 


WEATHER. 


portion  of  the  outside  bounding  line  of  the  cloud-area 
which  is  dotted  shows  where  observation  gave  the  end  of 
cloud  and  appearance  of  blue  sky.  The  portion  where 
the  single  shading  ends  without  a  dotted  line  merely 
shows  where  observations  ceased,  and  that  the  cloud 
extended  to  some  unknown  distance  beyond  these  limits. 
So  far  for  the  interpretation  of  the  relations  of  the 


FIG,  34. — Diurnal  variation  of  rain  and  cloud  in  a  cyclone. 

diurnal  variations  which  we  find  in  meteograms  to  the 
facts  concerning  the  nature  of  weather  which  we  derive 
from  synoptic  charts;  but  we  must  now  consider  some  of 
the  more  minute  phenomena  of  cyclones,  etc.,  which  can 
only  be  learnt  from  meteograms  and  from  verbal  descrip- 
tions of  the  sequence  of  weather  on  each  day. 

We  have  already  explained  the  broad  features  of  the 
sequence  of  blue  sky,  clouds,  rain,  back  to  blue  sky  again, 


BAROGRAMS,  THERMOGRAMS,  METEOGRAMS.  177 

in  a  cyclone ;  but  it  is  obvious  that,  as  the  stations  from 
which  the  materials  for  making  synoptic  charts  are 
derived  are  rarely  less  than  from  eighty  to  a  hundred  miles 
apart,  many  of  the  details  of  a  cyclone  are  lost.  For 
instance,  in  practice,  we  rarely  get  more  than  one  or  two 
stations  to  report  halo  at  the  same  time,  and  from  that 
we  could  never  deduce  the  shape  of  the  halo-forming 


FIG.  35. — Diurnal  variation  of  rain  and  cloud  in  a  cyclone. 

portion  of  a  cyclone.  But  when  we  observe  in  a  great 
number  of  cases  that  halo  sky  rarely  lasts  long,  and  is 
exclusively  formed  in  front  of  the  regular  cloud-area,  then 
we  conclude  that  the  ring  of  halo  sky,  such  as  we  have 
marked  in  our  diagram  of  cyclone-prognostics,  is  very 
narrow.  The  reason  is,  that  as  the  cloud-area  is  propa- 
gated at  the  same  rate  as  the  cyclone,  which  we  may 
take  at  twenty  miles  an  hour,  and  that  as  a  halo  usually 

N 


178  WEATHEE. 

lasts,  say,  only  half  an  hour,  therefore  the  width  of  the 
halo-ring  will  not  be  much  more  than  ten  miles.  Of 
course,  if  charts  could  be  constructed  for  every  hour  of 
the  day,  and  at  stations  only  five  or  ten  miles  apart,  there 
is  nothing  we  learn  from  meteograms  which  we  could 
not  also  derive  from  charts ;  but,  as  such  observations  are 
impracticable,  it  is  of  the  utmost  importance  to  know 
precisely  how  the  continuous  trace  of  instruments  at  any 
one  station  can  be  collated  with  the  intermittent  observa- 
tions at  widely  scattered  localities. 

The  most  striking  example  of  the  value  of  meteograms 
in  building  up  the  nature  of  a  cyclone  is  found  in  the 
phenomena  of  the  trough.  These  are  confined  to  a  line 
only  a  mile  or  two  wide,  and  it  would  be  utterly  impos- 
sible from  charts  alone  ever  to  learn  the  significance  of 
the  turn  of  the  barometer.  We  might  look  at  fifty  charts 
of  different  cyclones,  and  it  might  happen  that  the  trough 
was  not  actually  passing  over  any  observing-station  in 
any  one  of  them. 

But  if,  in  a  large  number  of  cyclones,  it  is  found  that 
whenever  the  barometer  turns  to  rise  there  is  a  squall, 
this,  being  independent  of  the  time  of  day,  must  be 
referred  to  that  part  of  the  cyclone  for  its  origin ;  and 
since  this  phenomenon  occurs  at  all  places  over  which  the 
cyclone-trough  passes,  however  distant  from  its  centre,  if 
a  synoptic  chart  could  be  made  with  a  large  number  of 
stations  close  together,  a  line  of  squalls  would  be  seen 
under  the  trough  of  the  cyclone,  marking  all  the  points 
at  which  the  barometer  turned  to  rise  simultaneously. 

This  inference  may  be  derived  either  by  taking  the 
history  of  the  passage  of  a  single  cyclone,  and  observing 


BAROGRAMS,   THERMOGRAMS,  METEOGRAMS.  179 

that  a  squall  was  associated  with  the  trough  at  every 
station,  or  else  by  observing  at  any  one  station  that  in 
every  cyclone  which  passed,  the  trough  and  a  squall  came 
together.  The  latter  deduction  hangs  on  the  assumption 
that  in  a  great  number  of  cyclones  no  two  need  be  sup- 
posed to  pass  at  the  same  distance  from  the  station ;  so 
that,  to  a  certain  extent,  a  large  number  of  different 
sections  of  a  single  cyclone,  and  a  large  number  of  single 
sections  of  different  cyclones,  give  the  same  result. 

The  method  of  the  meteorologist  is,  in  fact,  analogous 
to  that  of  the  microscopist,  who  builds  up  his  picture  of 
the  organs  of  an  animal  by  taking  a  series  of  their 
sections,  across  any  portion  of  it. 

There  are  many  other  deductions  which  can  be  made 
as  to  how  the  flexure  of  barogranas  indicate  the  nature  of 
the  gradients  that  are  being  propagated  over  any  place ; 
and  as  to  how  squalls  and  thunderstorms,  and  even  single 
gusts,  each  leave  their  characteristic  mark  on  a  baro- 
graphic  trace,  which  can  be  read  off  any  time  afterwards 
by  a  practised  observer.  We  have  already  explained 
how  some  of  the  fluctuations  of  a  thermograph  tell 
their  own  story  about  cold  showers,  or  passing  clouds,  and 
many  other  deductions  can  be  made  from  these  traces. 
We  could  also  point  out  how  wind- traces  reflect  each 
fitful  gust  in  their  own  appropriate  manner ;  and  also  how 
minute  details  of  the  relation  of  wind-direction  to  cyclone 
and  anticyclone  centres,  as  well  as  minor  diurnal  varia- 
tions both  of  force  and  direction,  can  be  deduced  more 
accurately  from  anemograms  than  from  charts.  The 
consideration  of  these  is,  however,  unsuitable  for  an 
elementary  work,  and  the  object  of  this  chapter  so  far 


180  WEATHER. 

will  have  been  attained  if  we  have  conveyed  to  the  reader 
a  clear  idea  that  observations  at  any  one  station  give  a 
section  of  the  weather-changes  which  are  shown  in  plan 
on  successive  synoptic  charts  ;  and  that  each  self-recording 
instrument  writes  in  its  own  language,  and,  as  it  were, 
in  its  own  alphabet,  the  history  of  the  weather  for  every 
day. 

DESCRIPTIVE,  OR  NON-INSTRUMENTAL,  RECORDS. 

So  far  we  have  discussed  the  significance  of  instru- 
mental records ;  but,  however  skilfully  we  may  read  those 
written  traces,  it  is  evident  that  there  is  still  a  great 
deal  of  weather  about  which  they  tell  us  nothing.  No 
mechanical  registration  of  pressure,  temperature,  or  wind 
can  ever  make  up  for  the  want  of  a  good  verbal  descrip- 
tion of  weather-sequence.  No  instrument  can  picture  to 
us  the  various  ways  in  which  a  blue  sky  can  become  over- 
cast ;  whether  the  blue  grows  gradually  pale  and  sickly, 
or  whether  great  snaky-looking  clouds  seem  irresistibly 
to  embrace  the  whole  heavens.  Neither  can  it  describe 
the  delicate  distinctions  which  our  senses  enable  us  to 
perceive  in  the  way  the  wind  blows.  Our  eyes  tell  us  at 
a  glance  that  a  south-west  wind  raises  a  long  sea,  while 
a  nor'-wester  rakes  the  surface  of  the  ocean  into  lines  of 
foam  ;  and  that  the  fitful  gusts  of  an  impending  shower 
drive  little  eddies  along  the  dusty  road. 

In  like  manner,  no  short  cloud-symbols,  such  as 
detached  cloud,  overcast,  misty,  or  even  the  more  detailed 
words — cirrus,  cumulus,  etc.,  can  ever  give  more  than  a 
lifeless  picture  of  the  sky  as  we  know  it. 


BAROGRAMS,   THERMOGRAMS,   METEOGRAMS.  181 

The  old  myth-makers  excelled  in  their  descriptions 
of  weather.  In  their  own  peculiar  figurative  language 
we  see  reflected  a  vivid  picture  of  cloud  and  thunderstorm 
which  we  can  scarcely  match  in  the  more  sober  verbiage 
of  modern  times.  The  Greek  poets  knew  the  difference 
between  the  beneficent  diurnal  winds  which  sprang  up  at 
dawn  and  the  dangerous  blasts  of  an  approaching 
thunderstorm  ;  and  never  mistook  the  wind  which  sighed 
among  the  pinetops  for  the  north-westerly  squalls  which 
tumbled  the  trees  over  the  cliffs. 

All  that  instrumental  traces  could  tell  of  this  would 
be  deduced  from  seeing  if  the  velocity-trace  had  some 
connection  with  the  time  of  day,  or  if  it  was  fitful,  and 
that  the  direction-trace  was  also  unsteady;  or  whether 
some  directions,  such  as  the  north-west,  were  associated 
with  higher  velocities  than  others. 

On  the  other  hand,  instruments  not  only  give  precision 
to  the  general  impressions  derived  from  the  senses,  which 
alone  a  savage  can  receive  ;  but  also  enable  us  to  discover 
some  changes  which  our  perceptions  alone  could  never 
detect.  For  instance,  by  measuring  heat-curves  we  can 
calculate  the  ordinary  amount  of  daily  range,  and  com- 
pare the  value  in  London  with  that  in  Berlin,  or  New 
York ;  and  we  can  also  draw  deductions  from  certain 
bends  in  the  temperature-curve  which  would  never  have 
entered  into  the  head  of  semi- civilized  man.  In  like 
manner,  there  is  in  England  a  small  increase  of  the  wind- 
velocity  about  1  a.m.  which  has  some  scientific  interest, 
but  which  certainly  would  not  have  been  discovered 
without  instrumental  appliances. 

But,  in  addition,  the  invention  of  the  barometer  has 


182  WEATHER. 

given  us  another  sense — that  is  to  say,,  the  appreciation 
of  the  varying  weight  of  the  atmosphere,  which  was 
denied  to  our  ancestors ;  and  this  book  is  the  answer  to 
the  question  how  much  weather-knowledge  can  be  derived 
from  observation  of  that  instrument. 

It  will  be  found  a  distinguishing  feature  of  this  work 
that  we  have  endeavoured  to  describe  the  weather  in 
different  shapes  of  isobars,  so  far  as  possible,  in  the 
language  of  popular  prognostics.  This  language,  while 
it  contains  many  survivals  of  mythic  speech,  is  still  in 
current  use,  and  gives  a  much  more  accurate  picture  of 
weather  than  more  formal  language.  It  is  far  more  life- 
like to  talk  of  a  cyclone-front  as  dirty  and  muggy  than 
to  report  sky  overcast,  humidity  ninety-eight  per  cent. ; 
or  to  say  that  the  sun  "  draws  water  "  in  straight  isobars 
rather  than  c.  9  stratus  (sky  nine-tenths  overcast,  stratus- 
cloud).  We  use,  in  fact,  the  phraseology  of  popular 
weather-lore  to  translate,  as  it  were,  the  indications  of 
instrumental  readings  into  the  language  of  common  life. 

At  the  same  time,  we  have  already  examined  most 
carefully  the  minuter  fluctuations  of  some  instrumental 
traces,  and  in  various  chapters  we  shall  investigate  the 
precise  significance  of  the  results  of  various  arithmetical 
calculations  which  can  be  made  from  the  numerical  values 
derived  from  thermograms,  etc. 

The  problems  which  the  meteorologist  has  to  solve  are 
so  complex  and  varied  that  he  cannot  afford  to  dispense 
with  any  possible  assistance  from  whatever  quarter;  and 
our  endeavour  has  been  to  convey  to  the  reader  the  results 
of  every  line  of  investigation,  and  to  collate  the  old  and 
new  meteorology  into  one  compact  science. 


(     183    ) 


CHAPTER   VI. 

WIND    AND    CALM. 

IN  the  preceding  chapters  we  have  ouly  stated  that  in 
most  cases  the  force  or  velocity  of  the  wind  is  roughly 
proportional  to  the  closeness  of  the  isobars ;  but  we  shall 
now  go  into  the  details  of  the  subject,  and  give  the  actual 
numbers  which  connect  wind  and  gradients.  We  shall 
then  point  out  various  sources  of  variation  which  prevent 
us  from  laying  down  any  law  of  wind  with  mathematical 
accuracy,  and  carry  out  the  same  idea  with  reference  to 
the  relation  of  the  angle  between  the  direction  of  the 
wind  and  the  lie  of  the  isobars.  After  that  we  shall 
extend  these  and  other  general  principles  of  wind  to  the 
southern  hemisphere,  and  conclude  with  a  few  general 
reflections  on  the  subject. 

GRADIENTS. 

The  relative  closeness  of  any  two  isobars  is  not 
measured  by  the  number  of  miles  between  them,  but  by 
the  steepness  of  the  barometric  slope  which  they  indi- 
cate. For  instance,  suppose  that  two  isobars  differ  by 
0'2  in.  (5  mm.)  of  barometric  level — say  29'7  and  29'9  in. 


18-fc  WEATHER. 

(755  and  760  mm.) — we  do  not  measure  their  relative 
proximity  by  saying  that  they  are  thirty  or  ninety 
miles  apart,  but  we  think  of  the  barometric  slope  with 
a  rise  of  two-tenths  of  an  inch  (5  mm.)  in  either  thirty 
or  ninety  miles.  Then,  to  reduce  this  to  a  common 
standard,  we  take  a  uniform  distance — in  England  fifteen 
nautical  miles,  or  seventeen  statute  miles — and  calculate 
how  many  hundredths  of  an  inch  the  barometer  would 
rise  in  those  fifteen  miles ;  that  is  to  say,  we  treat  the 
barometric  slope  like  the  slope  of  a  hill,  which  is  uni- 
versally estimated  by  saying  that  the  latter  rises  so  many 
feet  in  a  mile. 

The  slope  between  two  isobars  is  called  the  barometric 
gradient,  and,  of  course,  it  is  measured  square  or  at  right 
angles  to  the  isobars,  just  in  the  same  way  that  we 
measure  the  slope  of  a  hill  between  two  contour  lines. 

For  instance,  suppose  that  in  Fig.  36  the  line  A  B, 
drawn  square  to  the  isobars,  is  thirty  nautical  miles  long, 
and  that  the  isobars  denote  differences  of  two-tenths  or 
twenty-hundredths  of  an  inch ;  then  the  rise  in  filteen 
nautical  miles  would  be  ten-hundredths  of  an  inch  ;  and 
we  should  say  that  there  was  a  gradient  of  ten  between 
the  two  stations  A  and  B.  If  the  distance  between  the  same 
two  isobars  at  c  and  D  was  ninety  miles,  the  gradient  over 
an  observer  at  E  would  only  be  0*2  X  100  X  £§•  =  3'3 ; 
and  this  last  number  Mould  be  the  required  gradient. 

In  practice  we  too  often  come  across  the  error  of  taking 
the  difference  of  pressure  at  two  places,  F  and  G,  and 
calculating  the  gradient  from  the  distance  in  miles  between 
them.  This  always  gives  a  smaller  gradient  than  the 
real  one,  for  the  line  of  a  gradient  is  always  the  shortest 


WIND   AND   CALM. 


185 


line  which  can  be  drawn  between  two  isobars.  In  this 
case  G  H,  not  G  F,  is  the  proper  line  to  measure  for  the 
gradient  at  G.  The  best  method  for  measuring  gradients 
on  a  synoptic  chart  is  to  take  the  station  E,  whose  gradient 


.29.7  in. 


755 


FIG.  36. — Barometric  gradients. 

is  required,  and  then  to  draw  the  line  c  D  through  E  on 
the  map,  square  to  the  isobars,  and  measure  its  length  on 
the  scale  of  the  chart.  This  is  much  quicker  and  more 
accurate  than  trying  to  find  two  places  which  are  nearly 
square  to  the  isobars.  Gradients  as  thus  measured  are 
rarely  higher  than  four  or  five  in  Great  Britain,  though 
much  higher  values  than  these  are  occasionally  recorded 
in  very  severe  storms.  In  a  general  way,  it  may  be  said 
that  gradients  are  slight  or  moderate  when  below  one, 
and  steep  when  above  two.  In  Europe  gradients  are 
measured  by  the  number  of  millimetres  of  barometric 
difference  in  one  degree  of  a  great  circle  of  the  earth ; 
that  is,  in  sixty  geographical  miles.  For  any  slope  the 
numbers  are  nearly  identical  with  those  in  England,  for 
one  millimetre  very  nearly  equals  0*04  in.,  and  fifteen 
nautical  miles  are  exactly  equal  to  one  degree. 


186 


WEATHER. 


RELATION  OF  VELOCITY  TO  GRADIENT. 

But  it  may  be  interesting  to  see  what  the  velocity  of 
the  wind  actually  is  for  any  given  gradient. 

The  following  are  the  numbers  obtained  by  Messrs. 
Whipple  and  Baker  at  Kew,  near  London,  both  in  English 
and  metrical  equivalents : — 


. 
Gradients 
per  fifteen 
nautical  miles. 

Wind-velocity 
in  miles 
per  hour. 

Gradient  in 
millimetres  per 
degree  of  latitude. 

Velocity 
in  metres 
per  second. 

0-2 

50 

•203 

2-23 

0-5 

7-0 

•508 

3-13 

0-7 

7-5 

•711 

3-35 

1-0 

9-2 

1016 

4-11 

1-2 

11-6 

1-219 

5-19 

T5 

12-6 

1-524 

5-63 

1'7 

15-0 

1-727 

6-70 

2-0 

16-5 

2-032 

7'38 

2-2 

19-1 

2-235 

8-54 

2-5 

22-0 

2-540 

983 

27 

22*2 

2-743 

9-92 

3-0 

25-2 

3-048 

11-40 

Loomis  has  arrived  at  the  following   values   in  the 
United  States :— 


Gradient 

Velocity 

Gradient 

Velocity 

millimetres 

in  metres 

millimetres 

in  metres 

per  degree. 

per  second.                     per  degree. 

per  second. 

!i 

2-09 

7*20 

2-80 

10-64 

2-31 

8-18 

2-90 

11-09 

2-48 

9-03 

8-08 

11-44 

2-61 

9-66 

3-36 

11-80 

2-72 

10-28 

3-73 

12-20 

WIND  AND  CALM.  187 

These  agree  very  fairly  well  with  the  British  observa- 
tions. 

In  the  Atlantic,,  Professor  Loomis  finds  that  for  the 
same  gradient  the  velocity  of  the  wind  is  forty  per  cent, 
greater  than  in  the^United  States.  This  is  doubtless  due 
to  the  influence  of  a  certain  number  of  sheltered  stations 
among  the  land-observatories.  Nearly  every  place  feels 
some  winds  more  than  others,,  and  will  therefore  some- 
times report  comparatively  little  wind  for  a  considerable 
gradient.  Thus,  when  the  results  are  averaged,  the  mean 
values  will  be  lower  than  if  every  observer  was  equally 
exposed  to  all  winds,  as  in  the  open  sea. 

Wind  is  much  stronger  for  the  same  gradients  in  the 
tropics  than  in  higher  latitudes.  In  the  Indian  Ocean, 
especially,  the  north-east  and  north-west  monsoons-  blow 
steady  with  a  gradient  that  would  give  variable  winds  in 
temperate  regions ;  and  the  violence  of  the  south-west 
monsoon  is  out  of  all  keeping  with  the  steepness  of 
gradient  according  to  European  experience.. 


VARIATIONS  IN  TELOCITY  AND  GRADIENT;. 

There  are  various  sources  of  variation  from  these 
general  laws  of  the  relation  of  velocity  to  gradient,  some 
of  which  only  can  be  explained  in  an  elementary  work 
like  the  present.  In  Great  Britain  it  is  found  that,  for 
any  given  (moderate)  gradient,  winds  from  north  and  east 
points  are  stronger  than  those  from  south  and  west 
points.  For  instance,  Ley  has  found  at  Kew  the  follow- 
ing differences: — 


188 


WEATHER. 


Gradient  per 
fifteen  nautical 
miles. 

Velocity  in  miles 
for  winds  from 
S.S.E.  by  S. 
to  N.W. 

Velocity  in  miles 
for  winds  from 

N.N.W.  by  N. 
to  S.E. 

•006 

4-14 

6-89 

•009 

6-41 

8-63 

•012 

8-37 

10-93 

•015 

11-21 

14-27 

•016 

13-56 

16-98 

reason  -why  the  mean  of  these  does  not  agree  with 
the  mean  velocity  for  the  same  gradients  as  deduced  by 
Mr.  Whipple  for  the  same  station  is  readily  explained. 
The  latter  takes  all  winds  from  all  directions  for  the 
same  gradient,  and  averages  them  up  together.  Mr.  Ley 
separates  the  two  principal  directions ;  but,  as  one  direc- 
tion— the  south-west — is  much  more  frequent  than  the 
north-west,  his  numbers  would  have  to  be  weighted  pro- 
portional to  the  frequency  of  these  two  directions  to  give 
the  same  numbers  as  Mr.  Whipple. 

Local  variation  of  wind  is  too  obvious  to  need  much 
comment.  The  only  thing  we  have  to  consider  here  is 
how  it  affects  forecasting.  If  every  station  was  equally 
exposed  to  every  quarter,  it  might  be  possible  to  issue 
forecasts  in  which  the  amount  of  wind  recorded  by  any 
instrument  might  be  approximately  indicated ;  but  when 
we  have  to  deal  with  a  gale  which  begins  in  the  south- 
east and  works  round  to  north-west,  it  is  manifest  that  we 
can  only  state  the  probable  amount  of  wind  in  general 
terms,  as  no  place  is  equally  influenced  by  wind  from 
these  two  quarters. 

The  relation  of  diurnal  variation  of  wind- velocity  to 


WIND  AND  CALM.  189 

gradient  has  already  been  discussed  in  our  chapter  on 
Meteograms ;  and  other  diurnal  phases  of  wind,  such  as 
land  and  sea  breezes  or  valley  winds,  will  be  most  con- 
veniently taken  in  our  chapter  on  Diurnal  Variations  of 
Weather.  But,  in  temperate  regions,  by  far  the  most 
important  elements  of  disturbance  in  the  simple  relation 
of  wind  to  gradient  are  squalls  and  thunderstorms.  In 
both  of  these  the  barometer  usually  rises  suddenly,  some- 
times as  much  as  one-tenth  of  an  inch,  from  causes  which 
are  at  present  obscure.  And  in  both  we  find  angry, 
violent  gusts,  which  bear  no  relation  whatever  to  isobaric 
gradients.  Many  of  the  discordant  observations  on  this 
subject  are  doubtless  due  to  want  of  care  in  distinguishing 
one  kind  of  wind  from  another. 

Loomis  has  called  attention  to  the  total  want  of 
accordance  between  wind  and  gradient  which  he  has 
found  during  the  "  northers  "  of  New  Mexico,  and  the 
author  has  found  that  in  the  "  nortes "  of  Panama  the 
wind  also  is  quite  disproportionate  to  the  gradient. 
Finley  has  also  discovered  in  the  United  States  what  are 
called  "  straight-line  gales,"  or  long  streaks  of  wind,  two 
or  three  miles  across,  blowing  at  the  rate  of  sixty  to  eighty 
miles  an  hour,  and  extending  over  eighty  or  one  hundred 
miles  in  length.  These  appear  on  the  side  of  a  cyclonic 
depression,  at  some  distance  from  the  centre,  and  are  not 
associated  with  any  deflection  of  the  isobars.  Other 
winds  that  are  not  directly  associated  with  isobaric 
gradients  have  been  noticed  in  other  parts  of  the  world, 
and  we  are  therefore  led  to  the  conclusion  that,  though 
the  great  wind-circulation  of  the  atmosphere  is  related  to 
isobars,  still  there  are  some  winds  that  are  impelled  by 


100  WEATHER. 

other  causes  than  those  which  develop  isobars ;  and  for 
the  sake  of  classification  we  will  call  them  generically 
"  non-isobaric  winds."  They  are  most  probably  connected 
with  what  we  have  before  alluded  to  as  non-isobaric 
rains, 

We  cannot  say  what  is  the  origin  of  the  wind  in 
thunderstorms  and  non-isobarie  winds,  but  it  is  certain 
that  the  cause  is  quite  different  from  that  in  cyclones. 
We  must  therefore  take  care,  in  talking  about  wind,  not 
to  mix  up  two  kinds  which  really  have  little  in  common. 
From  all  this  we  see  the  very  fallacious  results  which 
come  of  trusting  blindly  to  instruments,  and  also  that 
any  statistical  values  which  are  derived  from  mixing  up 
various  sorts  of  wind  can  only  give  rise  to  discordant 
deductions. 

We  may  also  remark  that  merely  saying  that  a  storrn 
blew  with  such  a  force  or  velocity  tells  us  very  little 
either  of  the  true  character  of  the  wind  or  of  the  amount 
of  destruction  which  the  gale  might  cause.  An  instru- 
mental record  of  forty  miles  of  wind  in  an  hour  may  be 
made  up  either  of  a  steady  weight  of  wind,  or  of  violent 
gusts  alternating  with  quieter  intervals.  The  damage 
clone  in  the  latter  case  would  many  times  be  greater  than 
in  the  former. 

Then  there  are  many  minute  differences  in  the  way  of 
blowing  which  instruments  cannot  even  detect.  We  all 
know  that  most  chimneys  smoke  more  with  an  east  wind 
than  with  a  west  one.  We  have  also  just  shown  that 
the  velocities  of  these  two  winds  is  not  the  same  for  the 
same  gradients. 

It  has  -been  suggested,  with  a  great  deal  of  probability, 


WIND  AND   CALM.  191 

that  the  difference  may  be  due  to  the  wind  not  blowing 
horizontally,  and  that  east  winds  are  perhaps  directed 
a  little  downwards.  Another  very  striking  phase  of  wind 
is  the  difference  between  the  kind  of  sea  raised  by  -the 
south-west  gale  in  front  of  a  cyclone  and  the  north-wester 
in  rear.  The  first  raises  a  high  sea  with  only  a  moderate 
amount  of  white  water,  while  the  latter  rakes  the  surface 
of  the  ocean  into  long  streaks  of  foam.  There  are  other 
reasons  for  believing  that  in  front  of  a  cyclone  the  wind 
is  rising,  while  in  rear  the  air-currents  have  a  slant  down- 
wards. If  so,  the  cold,  dry  clearness  of  north-westers  is 
readily  explained.  The  whirls  of  dust  that  precede 
some  kinds  of  rain  are  also  familiar  instances  of  the 
specific  character  which  belongs  to  different  winds. 


RELATION  OF  DIRECTION  TO  GRADIENT. 

We  will  now  consider  the  details  of  the  relation  of  the 
direction  of  the  wind  to  gradients  and  to  the  lie  or  trend 
of  the  isobar  conjointly. 

When  we  talk  of  gradient  only,  we  get  no  indication 
of  the  direction  of  the  wind,  for  the  barometric  slope  may 
face  in  any  direction  or  have  any  aspect.  Following  the 
analogy  of  barometric  gradients  to  hill-slopes,  we  will 
call  the  direction  towards  which  gradients  slope  the  aspect 
of  the  gradient,  so  as  to  keep  the  word  direction  for  wind. 
For  instance,  if  isobars  run  east  and  west  they  may  slope 
either  north  or  south,  or  we  might  say  that  the  aspect  of 
the  gradients  was  either  towards  the  north  or  the  south, 
just  as  we  should  talk  of  a  hill ;  or,  to  take- the  analogy 
of  geological  terms,  we  might  say  that  the  strike  of  the 


192  WEATHER. 

isobars  was  east  and  west,  but  that  the  gradients  dipped 
either  north  or  south. 

But  by  combining  the  idea  of  gradient  with  that  of 
aspect,  and  both  with  the  Buy  Ballot's  law,  we  see  at 
once  that  if  the  isobars  run  or  strike  east  and  west,  the 
general  direction  of  the  wind  will  be  westerly  if  the  aspect 
of  the  gradients  is  towards  the  north,  but  easterly  if  the 
aspect  is  to  the  south.  We  therefore  say  that  in  the 
former  case  we  have  gradients  of  such  a  value  for  westerly 
winds,  and  in  the  latter  gradients  of  such  an  amount  for 
easterly  winds.  This  holds  for  every  direction.  In 
Fig.  36  we  have,  as  before  explained,  a  gradient  of  ten 
between  A  and  B  for  velocity,  and  now  we  can  say  that 
the  gradient  is  also  for  north-westerly  winds ;  at  E  there 
is  a  gradient  of  3'3  for  south-westerly  winds.  By  this 
simple  method  of  expression,  whenever  we  see  a  synoptic 
chart,  we  can  calculate  at  once  both  the  probable  direction 
and  force  of  the  wind. 


INCLINATION  OF  WIND  TO  ISOBARS. 

Buy  Ballot's  law  does  very  well  for  the  general  sweep 
of  the  wind,  but  the  subject  is  capable  of  much  greater 
refinement.  The  acute  angle  between  the  direction  of  the 
wind  and  the  lie  of  the  isobar  is  called  the  inclination  of 
the  wind  to  that  isobar.  Taking  all  kinds  of  winds  and 
all  kinds  of  isobars,  Whipple  has  found  that  the  inclina- 
tion amounts  to  52°  at  Kew ;  while  Loomis  has  deduced 
an  angle  of  42°  in  the  United  States. 

But,  by  taking  the  inclination  of  the  wind  in  different 
shapes  of  isobars  and  different  portions  of  each  shape, 


WIND   AND  CALM.  19-3 

Ley,  Loomis,  Hildebrandson,  and  others,  have  arrived  at 
a  series  of  remarkable  generalizations  as  to  the  general 
circulation  of  the  atmosphere.  They  find  that  the  wind 
is  much  more  inclined  and  incurved  in  the  right  front  of 
a  cyclone  than  in  any  other  portion  ;  and  that  in  the  rear 
the  inclination  is  very  small,  if  not  occasionally  reversed 
— that  is  to  say,  a  little  outcurved. 

We  have  examined  the  details  of  these  cyclone 
surface-winds,  as  well  as  of  those  in  an  anticyclone  in 
our  chapter  on  Clouds.  There  we  treated  each  shape 
separately,  but  we  can  connect  both  in  a  very  striking 
manner  if  we  call  attention  to  some  general  values  obtained 
by  Loomis  from  observations  over  the  Atlantic  Ocean. 
Taking  an  ideal  cyclone,  with  an  adjacent  anticyclone, 
he  finds  that,  starting  from  the  anticyclone,  the  inclina- 
tion of  the  wind  to  the  isobar  begins  at  about  52°,  and 
then  gradually  decreases  to  25°  near  the  centre  of  a 
cyclone.  Of  course  this  is  a  generalized  case,  for  we  have 
shown  that  the  inclination  is  not  the  same  on  different 
sides  of  a  cyclone.  The  great  thing  to  remember  is  that 
in  every  shape  of  isobars  each  part  has  a  wind  velocity 
and  direction  of  its  own  relative  to  the  gradients. 

The  only  other  material  source  of  variation  is  diurnal. 
We  have  already  sufficiently  explained,  in  our  chapter- 
on  the  Meteograms,  that,  whatever  the  inclination  due  to 
any  part  of  any  shape  of  isobars  may  be,  the  diurnal 
variation  imposes  a  modification  on  that,  but  does  not 
alter  the  direction  due  to  general  causes.  Land  and  sea 
breezes  we  shall  discuss  in  our  chapter  on  Diurnal 
Variations  of  Weather. 


194  WEATHEK. 


CALMS. 

We  have  already  stated  that  calms  are  the  product 
of  no  barometric  gradient.  The  most  persistent  calms 
are  found  in  the  "  doldrums,"  or  the  col  of  low  pressure 
near  the  equator  between  the  north-east  and  south-east 
trade  winds  all  over  the  world. 

In  temperate  regions  the  most  persistent  calms  are 
near  the  centres  of  stationary  anticyclones ;  but  more 
short-lived  calms  are  found  in  the  centres  of  cyclones, 
along  the  crest  of  wedges,  and  in  cols. 

We  do  not  think  it  necessary  to  give  any  special 
examples  of  either  gales  or  calms,  for  they  are  abundantly 
illustrated  by  numerous  charts  in  the  course  of  the 
work;  we  need  only  call  attention  to  Figs.  65  and  66 
of  south-westerly  gales  in  Great  Britain,  to  Figs.  77  and 
78  of  easterly  gales,  and  to  Figs.  22  and  24  of  calms.  . 


WINDS  IN  THE  SOUTHERN  HEMISPHERE. 

So  far  we  have  confined  our  attention  to  winds  in 
the  northern  hemisphere  only ;  now,  however,  that  we 
thoroughly  understand  the  nature  of  wind  in  that 
hemisphere,  we  can  easily  follow  the  modifications  which 
occur  south  of  the  equator. 

The  great  general  principles — that  every  shape  of 
isobars  has  a  distinctive  wind;  that  cyclones  incurve, 
while  anticyclones  outcurve ;  that  the  velocity  is  mainly 
determined  by  the  gradient,  and  also  the  relation  of 
diurnal  to  general  winds — are  the  same  in  both  hemi- 


WIND  AND   CALM.  195 

spheres.  What  does  differ  is  that  portion  of  Buy  Ballot's 
law  which  gives  the  position  of  the  nearest  low  pressure 
to  an  observer  who  turns  his  back  to  the  wind.  For  the 
southern  hemisphere  the  law  is  as  follows : — stand  with 
your  back  to  the  wind,  and  pressure  will  be  lower  on 
your  right  hand  than  on  your  left.  This  is  exactly  the 
converse  of  what  holds  north  of  the  equator. 

As  a  necessary  consequence  of  this,  the  surface-wind 
will  rotate  round  a  cyclone  or  anticyclone  in  the  opposite 
manner  to  what  it  does  in  the  northern  hemisphere. 
That  is  to  say,  a  cyclone  rotates  in  the  direction  of  the 
motion  of  the  watch-hands,  but  is  incurved;  while  the 
anticyclone  turns  against  the  watch-hands,  but  is  still 
outcurved,  as  in  Europe.  The  general  circulation  of  the 
upper  currents  is  exactly  analogous  to  that  of  the  northern 
hemisphere,  being  nearly  parallel  to  the  isobars  at  the 
level  of  the  lower  cumulus,  and  more  or  less  outwards  at 
the  higher  cirrus  level  in  cyclones,  and  inwards  in  anti- 
cyclones. 

For  this  reason,  the  vertical  succession  of  the  upper 
currents  is  contrary  to  that  of  Europe.  There  the  upper 
currents  always  come  successively  more  and  more  from  the 
left  as  you  stand  with  your  back  to  the  wind ;  whereas 
they  will  come  more  and  more  from  the  right  in  the 
southern  hemisphere.  For  instance,  if  the  surface-wind 
was  south,  in  Europe  the  lowest  clouds  would  be  south- 
south-west,  the  next  layer  south-west,  and  the  highest 
cirrus  perhaps  from  west.  Whereas  in  Australia,  with 
the  same  surface-wind  from  south,  the  successive  upper 
currents  would  come  from  south-south-east,  south-east, 
-and  east  respectively. 


196  WEATHER. 

The  diurnal  variation  of  wind-velocity  will  be  fully 
discussed  in  our  chapter  on  that  subject. 

The  sequence  of  wind  as  a  cyclone  drifts  past  an 
Australian  station  would  be  different  from  that  of  Europe. 
Anywhere  south  of  the  line,  the  wind  goes  round  by  the 
north  if  the  centre  passes  south,  and  round  by  south  if 
the  centre  passes  north,  of  the  observer,  which  is  exactly 
the  converse  of  what  happens  in  Europe.  Mr.  Ringwood 
has  pointed  out  that  we  can  express  both  cases  by  one 
general  law,  if  we  say  that  in  both  hemispheres  the  wind 
goes  round  by  the  polar  side  when  the  centre  passes  on 
the  equatorial  side  of  the  station,  and  by  the  equatorial 
side  when  the  centre  passes  on  the  polar  side. 

This  can  all  be  better  illustrated  by  a  few  actual 
examples  than  by  generalized  diagrams,  the  more  so  as 
the  figures  can  then  be  made  to  show  some  other  in- 
teresting phenomena. 

In  Fig.  37  we  give  an  example  of  a  violent  cyclone 
which  blew  in  the  Indian  Ocean  on  February  13,  1861, 
between  10°  and  20°  south  latitude,  and  about  80°  east  of 
Greenwich,  as  deduced  by  Mr.  Meldrum,  of  the  Mauritius. 
The  general  nature  of  the  rotation  of  the  wind  with  the 
direction  of  the  watch-hands  will  be  very  obvious,  but  we 
should  note  that  the  incurvature  in  most  places  is  very 
considerable,  especially  to  the  west  of  the  centre.  To  the 
south  of  the  centre  the  wind  was  south-east ;  to  the  west, 
south-west ;  to  the  north,  north-west ;  and  from  north- 
north-west  to  .north-east  on  the  eastern  edge  of  the 
cyclone.  The  four  feathered  arrows  denote  a  wind  of 
hurricane  force,  and  there  is  nothing  in  the  steepness 
of  the  gradients  to  suggest  such  high  velocities. 


WIND  AND   CALM. 


197 


The  path  of  the  cyclone  is  marked  by  dated  crosses, 
and  we  see  that  the  motion,  as  usual  in  these  latitudes, 
was  very  slow.  From  the  12th  to  the  13th  the  travel 
was  a  very  little  towards  the  east,  at  a  rate  of  hardly 
three  miles  an  hour  ;  after  that,  the  path  was  irregularly 
towards  the  south,  at  a  rather  higher  speed. 

We  have  introduced  this  example  from  the  South 


FIG.  37 — Tropical  hurricane  (south  of  the  equator). 

Indian  Ocean  partly  to  show  that  in  all  principal 
characteristics  a  tropical  does  not  differ  from  an  extra- 
tropical  cyclone ;  but  we  shall  understand  the  antithesis 
of  the  wind-sequence  far  better  from  an  Australian 
example,  because  the  conditions  of  weather  in  that 
country  are  more  similar  to  those  of  Europe  or  the 
United  States  than  those  of  the  lower  latitudes. 


198 


WEATHER. 


In  Figs.  38  and  39  we  give  the  wind  and  isobars  for 
Australia  on  November  20  and  21,  1884.  For  these  we 
are  indebted  to  Mr.  Ellery,  of  the  Government  Observatory 
at  Melbourne.  We  find  in  the  first  chart  (Fig.  38)  that 
the  highest  pressure  is  over  Queensland,  and  that  a 
moderate-sized  cyclone  covers  the  Australian  Bight.  The 
wind  rotates  round  this  in  the  usual  manner  of  the 


20. 1 1. 8 j. 


jo.o 
29-9 


2.9-9 


FIG.  38. — Cyclone  (Australia). 

hemisphere,  being  north  and  north-east  in  front,  and  south 
and  south-west  in  rear.  Land  and  sea  breezes  deflect  the 
winds  along  the  coast-line,  and  in  the  interior  of  the 
island  variable  winds  are  reported,  owing  to  the  slight 
gradients  which  are  there  present.  By  next  morning 
(Fig.  39)  only  the  fragment  of  the  cyclone  appears  to 
the  south  of  Tasmania ;  portions  of  two  anticyclones  lie 


WIND  AND   CALM. 


199 


over  the  north-east  and  south-west  corners  of  Australia ; 
and  a  sort  of  ill-defined  V-depression  runs  up  towards  the 
col  which  lies  between  the  two  anticyclones.  The  rotation 
of  the  wind  round  this  Y  is  from  north-east  and  north  in 
front,  to  south  and  south-west  in  rear,  so  that  the  wind- 
sequence  at  Melbourne  for  the  day  was  from  north-east 


21.11.84 


jo.o 


JO.  I 


jo.o 


-9-9 


29.6 


FIG.  39. — V-depression  (Australia). 

by  north  to  north-west  and  south-west.  In  London  the 
passage  of  the  same  isobars  would  have  been  associated 
with  a  shift  of  wind  from  south-east  by  south  to  south-west 
and  north-west. 


. 


GENERAL  REMARKS. 

We  may  conclude  this  chapter  with  a  few  general 
remarks  on  the  subject  of  wind. 


200  WEATHER. 

In  the  first  place,  let  us  notice  how  little  influence 
the  rate  of  a  cyclone's  motion  has  on  the  velocity  of  the 
wind.  All  that  we  know  for  certain  about  the  influence 
of  the  motion  of  a  cyclone  is  that  a  high  rate^increases 
the  general  intensity  of  the  wind  and  weather  every- 
where, but  that  it  does  not  prevent  the  centre  from  being 
calm,  or  the  wind  from  being  light  on  any  side  where  the 
gradients  are  slight. 

In  squalls  the  independence  of  the  velocity  of  wind  to 
that  of  the  squall  as  a  whole  is  still  more  curious.  The 
latter  may  be  travelling,  perhaps,  only  twenty  miles  an 
hour,  but  the  first  blast  may  come  at  the  rate  of  sixty 
miles  an  hour.  This  fact  we  must  consider  as  due  to  an 
impulse  being  propagated  which  induces  wind  of  such  a 
velocity,  but  not  as  due  to  wind,  or  a  gust,  moving  solidly 
over  the  earth's  surface.  Such  an  impulse  is  found  in 
the  trough  of  a  cyclone  or  V-depression. 

We  have  not  thought  it  necessary  to  give  the  general 
principles  of  the  dependence  of  wind-circulation  on  the 
earth's  rotation,  as  that  may  be  found  in  any  text-book 
of  physical  science.  The  modification  of  Halley's  old 
theory  of  north-east  and  south-west  winds,  which  has 
been  proposed  by  Professor  Ferrel,  has  been  universally 
adopted  all  over  Europe  and  the  United  States.  The 
theory  is  hardly  known  in  England,  and  is  too  mathe- 
matical for  this  work.  No  doubt  the  earth's  rotation  is 
the  real  cause  of  the  general  direction  of  circulation  in 
cyclones  of  either  hemisphere,  but  what  we  cannot  explain 
is  the  inclination  of  wind  to  the  isobars.  Theoretically, 
any  small  difference  of  temperature  should  set  up  a  wind 
from  the  cold  to  the  hot  area ;  but  we  have  seen  already, 


WIND   AND   CALM.  201 

and  shall  see  still  more  in  our  next  chapter  on  Heat  and 
Cold,  that  differences  of  temperature  even  over  large 
areas  have  wonderfully  little  influence  on  wind.  The 
most  that  local  differences  of  heat  and  cold  do  is  to  set 
up  local  breezes,  such  as  land  and  sea,  or  valley  winds. 
Then,  theoretically,  this  cold  wind  should  flow  nearly 
straight  toward  the  hot  area,  only  a  little  deflected  to  the 
right  or  left,  according  to  circumstances,  by  the  earth's 
rotation.  In  like  manner,  any  difference  of  pressure,  from 
the  high  to  the  low  barometer,  however  caused,  should 
draw  wind  nearly  straight.  But,  in  our  chapter  on 
Diurnal  Weather,  we  shall  find  some  land  and  sea  breezes 
which  blow  nearly  parallel  to  the  coast-line. 

On  the  other  hand,  if  we  look  at  a  cyclone  purely  as 
a  circulating  mass  of  air,  the  wind  should  be  parallel  to 
the  isobar,  perhaps  even  a  little  outcurved  from  centrifugal 
force.  Now,  in  practice  the  wind  is  always  incurved,  and 
the  depression  of  a  cyclone  is  certainly  not  caused  by 
centrifugal  force.  The  fiercest  wind  which  ever  blew 
would  only  depress  the  barometer  a  few  hundredths  of  an 
inch,  instead  of  which  we  find  depressions  of  two  inches 
and  more  with  no  wind  over  fifty  miles  an  hour.  This, 
of  course,  is  on  the  supposition  that  whirling  air  acts  like 
a  fluid. 

The  idea  has  been  suggested  that  the  friction  of  the 
wind  on  the  earth's  surface  is  the  cause  of  the  incurvature, 
and  that  without  friction  the  wind  would  be  parallel  to 
the  isobars,  as  we  find  it  at  the  level  of  the  lowest  cloud- 
layers.  It  is  extremely  probable  that  this  is  at  least 
partially  true,  for  several  experiments  can  be  devised 
with  whirling  water,  in  which  friction  of  small  particles 


202  WEATHER. 

on  the  bottom  does  cause  them  to  be  collected  in  the 
centre,  instead  of  being  thrown  out  to  the  edges  of  the 
vessel. 


KELATION  OF  FORCE  TO  VELOCITY. 

Lastly,  we  may  say  a  few  words  about  the  relation  of 
force  to  velocity.  The  velocity  of  wind  is  a  real  quantity, 
which  is  perhaps  capable  of  measurement  in  the  abstract, 
though  we  are  at  present  far  from  being  able  to  gauge  it 
accurately.  But  it  is  quite  certain  that  there  is  no  such 
thing  as  an  absolute  force  which  corresponds  to  a  given 
velocity.  According  to  the  theory  of  stream-lines,  when 
even  an  inelastic  fluid  meets  an  obstacle,  if  the  angles  of 
the  obstruction  do  not  break  the  continuity  of  the  fluid 
so  as  to  form  eddies  or  vortices,  the  same  amount  of 
pressure  which  is  imposed  on  the  body  by  the  first 
deflection  of  the  fluid  is  given  back  again  as  the  stream- 
lines of  the  fluid  close  up  behind  the  obstruction.  For 
instance,  if  a  ship  is  lying  at  anchor  in  a  current,  the 
same  amount  of  strain  which  the  current  causes  on  her 
cable  when  forced  asunder  by  the  bows,  is  given  back 
when  the  current  closes  in  behind  her  ;  so  that  the  total 
pressure  which  she  experiences  is  only  that  due  to  the 
friction  of  the  water  on  her  skin.  This  is,  of  course,  on 
the  supposition  that  her  lines  are  so  easy  that  they  do 
not  break  the  stream-lines  so  as  to  form  little  eddies  or 
vortices. 

Now,  the  same  thing  holds  with  wind.  If  we  put  up 
two  square  plates  of  different  sizes,  face  to  the  wind,  the 
pressure  on  each  is  not  proportional  to  the  area,  while  in 


WIND  AND  CALM.  203 

light  breezes  neither  will  record  anything.  The  reason 
is  that,  in  light  wind,  a  thin  mobile  fluid  like  air  can 
glide  round  even  the  sharp  angles  of  a  square  without 
forming  eddies,  and  as  there  is  no  vacuum  formed  behind 
the  plate,  there  is  no  pressure  recorded.  In  higher  winds, 
the  stream-lines  are  broken,  and  every  shape  and  every 
sized  plate  of  the  same  shape  form  a  different  series  of 
eddies  round  the  rim  of  the  obstacle.  Then  the  amount 
of  rarification  behind  the  various  plates  is  neither  identical 
nor  proportional,  and  therefore  every  shape  and  size  of 
anemometer  indicates  discordantly  at  every  different 
velocity. 

From  all  this  it  follows  that,  though  we  might  say 
that  the  pressure  on  a  board  one  foot  square  was  twenty 
pounds,  and  might  compare  this  force  with  that  on 
another  board  of  the  same  size  and  mounting,  we  should 
not  be  justified  in  saying  that  the  force  of  the  wind  was 
twenty  pounds  per  square  foot  in  the  abstract,  because  a 
board  ten  feet  square,  even  if  of  the  same  shape,  would 
have  given  a  different  number. 


204  WEATHER. 


CHAPTEK  VII. 

HEAT  AND  COLD. 

IN  this  chapter  we  purpose  to  go  a  little  more  into  the 
details  of  the  manner  in  which  changes  of  temperature 
are  produced.  What  are  the  causes  of  burning  heats  and 
hard  frosts ;  why  is  the  same  day  of  the  month  hot  in 
one  year  and  cold  in  another ;  why  at  the  same  season  do 
hot  and  cold  days  follow  one  another  without  any  apparent 
sequence ;  and  why  is  England  sometimes  warmer  than 
France,  though  the  latter  is  nearer  the  equator  ? 

All  these  questions  we  propose  to  answer,  and  to  point 
out  how  easily  they  can  be  explained  by  means  of  synoptic 
charts.  The  difficulties  of  getting  rid  of  annual  and 
diurnal  variations  have  tempted  many  meteorologists  still 
to  adhere  to  the  old  method  of  averages,  which  can  only 
lead  to  unsatisfactory,  if  not  to  delusive,  results. 

DIURNAL  ISOTHERMS. 

The  question  we  have  to  solve  is  this.  We  know  that 
the  sun  is  the  principal  source  of  all  heat,  and,  if  nothing 
disturbed  his  rays,  there  would  be  a  regular  diminution 


HEAT  AND   COLD.  205 

of  temperature  from  the  equator  to  the  pole,  which  we 
shall  call  a  thermal  slope.  Every  day,  as  the  earth 
turned  under  the  sun,  a  well-defined  wave  of  variation 
would  be  imposed  on  this  slope.  The  lines  which  mark 
out  this  deflected  slope  we  will  call  diurnal  isotherms. 
We  have  first  to  determine  what  the  shape  of  the  lines 
would  be  at  any  moment  over  the  globe,  and  then  how 
these  diurnal  isotherms  would  modify  the  appearance  on 
a  synoptic  chart  of  any  local  developments  of  heat  or 
cold.  If  we  see  on  a  chart  that  at  eight  o'clock  in  the 
morning  there  is  a  curious  patch  of  heat  in  front  of  a 
cyclone,  how  are  we  to  discover  how  much  is  due  to 
cyclonic  causes  and  how  much  to  diurnal  variations? 
In  fact,  how  can  we  prove  that  the  heat  is  due  to  the 
cyclone,  and  not  to  the  time  of  day  ? 

First,  to  form  a  conception  of  the  diurnal  distribution 
of  temperature  over  the  world  at  any  moment.  The 
author  has  shown  that  the  general  shape  of  the  isotherms 
in  any  latitude  would  be  like  the  lines  in  Fig.  40,  if 
there  was  a  uniform  thermal  slope  from  the  equator  to 
the  pole,  and  no  disturbing  influences,  such  as  unequal 
distribution  of  land  and  sea. 

The  diagram  gives  the  ideal  shape  of  the  isotherms 
at  any  moment.  Noon  is  placed  in  the  middle  of  the 
diagram  in  longitude  180°,  and  the  lines  represent  the 
diurnal  variation  in  the  latitude  of  each  isotherm. 
The  scale  on  the  right  of  the  diagram  is  degrees  of 
latitude ;  that  on  the  left,  degrees  of  temperature.  For  an 
ideal  diagram  we  have  supposed  that  the  general  thermal 
slope  from  the  equator  is  1°  of  temperature  for  1°  of 
latitude.  The  principle  on  which  the  diagram  is  formed 


206 


WEATHER. 


is  as  follows.  Suppose  that  in  latitude  20°  the  tempera- 
ture, is  60°  Fahr.  at  midnight,  and  that  by  6  a.m.  the 
temperature  has  fallen  to  59° ;  then  we  should  have  to  go 
one  degree  of  latitude  further  south  at  that  hour,  if  we 
want  to  follow  the  position  of  the  isotherm  of  60°.  If,  in 
our  undisturbed  world,  we  could  walk  round  the  earth 
in  any  latitude  in  twenty-four  hours,  the  line  marked  60° 

Longitude/ 


4.5 


90 


135' 


180 


225 


270 


315 


360 


\\ 


\\ 


6 


9 


N 


M 


H 


FIG.  40. — Diagram  illustrating  the  shape  of  diurnal  isotherms. 


on  the  chart  represents  what  our  journey  would  be  if  we 
wanted  to  keep  under  a  uniform  temperature  of  60°  for 
the  whole  day.  Starting  at  midnight  on  the  left  of  the 
diagram,  we  should  have  to  go  sixty  geographical  miles 
south  and  90°  east  of  longitude  by  six  o'clock  in  the 
morning.  Between  then  and  3  p.m.  we  should  have  to 
make  300  miles  of  northing  and  155°  of  easting,  if  we 


HEAT  AND   COLD.  207 

still  wished  to  keep  our  thermometer  at  60° ;  and  from 
then  till  the  second  midnight  we  should  have  to  make 
240  miles  of  southing  and  135°  of  easting,  to  follow  the 
isotherm  of  60.°  Observe  that  the  easting  has  to  be  ex- 
pressed in  degrees  of  longitude,  for  the  number  of  miles 
in  a  degree  varies  with  the  latitude.  The  diagram  is  also 
based  on  the  supposition  that  there  is  a  pretty  uniform 
isothermal  slope  from  the  equator  to  the  pole,  and  that 
the  diurnal  range  of  temperature  does  not  vary  much 
within  5°  or  10°  of  latitude. 

Then,  if  there  were  no  irregularities  caused  by  cyclones, 
or  the  unequal  heating  of  land  or  water,  the  diurnal  ther- 
inogram  in  every  place  would  be  very  similar  in  shape  to 
the  trace  of  any  of  the  isotherms  as  plotted  on  a  chart  if 
we  turn  longitude  into  time,  and  latitude  into  degrees  of 
heat  on  a  suitable  scale.  In  fact,  we  may  conceive  the 
curves  shown  in  the  diagram  to  sweep  round  the  world 
with  the  earth's  rotation,  and  suppose  that  the  rise  or  fall 
of  temperature  at  any  station  was  caused  by  the  passage 
of  this  shape  of  isotherms,  just  as  the  motion  of  the 
barometer  is  the  product  of  the  propagation  of  different 
shapes  of  isobars  over  any  place.  For  instance,  in  the 
diagram  (Fig.  40)  the  strong  horizontal  line  shows  the 
position  of  the  section  across  the  diurnal  isotherms  which 
is  propagated  over  any  station  in  latitude  20°  north. 
Starting  from  the  first  midnight  on  the  left  of  the  diagram, 
the  thermometer  would  mark  60°.  By  6  a.m.  the  mer- 
cury would  have  fallen  to  59°,  as  that  isotherm  descends 
to  latitude  20°  at  that  hour.  Between  6  a.m.  and  3  p.m. 
five  isotherms  are  propagated  over  the  station,  so  that  the 
instrument  would  register  64°  at  the  latter  hour.  Then, 


208  WEATHER. 

as  lower  isotherms  begin  to  pass  over  the  observer,  the 
temperature  would  fall  at  the  rate  shown  in  the  figure, 
till  60°  was  reached  again  by  the  second  midnight. 

How  DIURNAL  MODIFY  GENERAL  ISOTHERMS. 

Now,  assuming  this  typical  distribution  of  heat,  we 
can  readily  see  how  the  diurnal  range  of  temperature 
modifies  any  isotherms  which  we  find  on  a  synoptic  chart. 

But,  first,  let  us  define  the  aspect  of  the  thermal  slope 
on  the  map  of  the  world  as  the  direction  in  which  the 
gradients  look,  if  we  suppose  the  isotherms  really  to 
represent  relative  heights.  For  instance,  in  all  curves 
the  aspect  of  the  slope  in  the  morning  after  six  o'clock 
is  towards  the  north-west,  while  in  the  afternoon  it  is 
towards  the  north-east. 

Now,  suppose  that  at  any  hour  we  find  a  certain  shape 
of  isotherms  on  a  synoptic  chart :  these  lines  represent 
the  diurnal  isotherms  as  modified  by  local  radiation,  etc. ; 
or  we  may  say  that  we  have  on  the  map  temperature- 
distribution  due  to  radiation  or  cyclonic  causes  lying  on 
a  diurnal  thermal  slope.  Then,  so  long  as  the  direction 
or  aspect  of  the  diurnal  slope  does  not  vary,  the  shapes 
of  the  isotherms  will  not  alter ;  only  the  numbers  which 
are  attached  to  them  will  change.  That  is  to  say,  the 
propagation  of  a  uniform  slope  alters  the  level,  but  not 
the  shape,  of  the  isotherms. 

For  instance,  let  the  square  A  B  I  a,  in  Fig.  41,  repre- 
sent an  area  of,  say,  30°  latitude  by  30°  (two  hours)  of 
longitude,  anywhere  on  the  surface  of  the  earth,  and  let 
the  slanting  dotted  lines  mark  a  very  exaggerated  after- 


HEAT  AND  COLD. 


209 


5-0       49°     48°       4; 

•f6*       45" 

\           \           a 

\          '*•           \ 

\ 
\           %N           '• 

\ 

\           \ 

\          \ 

\           \           \ 

\ 

\            \           \ 

J  !  i 

\ 
\ 

FIG.  41. — Thermal  slope,  and  shape  of 
isotherms. 


noon  thermal  slope,  from  45°  to  50°  at  the  rate  of  1J°  per 
hour. 

Also,  suppose  that  the  wind,  etc.,  of  a  cyclone  within 
the  square  had  very  much  contorted  the  isotherms ;  the 
resulting  shape  would  be  compounded  of  the  cyclonic  dis- 
turbance lying  on  the 
simple  diurnal  slope 
shown  on  the  diagram. 

Now,  if,  while  the 
cyclone  stood  still,  and 
the  diurnal  thermal 
slope  was  propagated 
over  the  square  for  two 
hours,  then  the  only  B 
effect  would  be  to 
leave  the  shape  of  the 

isotherms  absolutely  unchanged,  but  to  make  each  line 
mark  3°  lower.  The  isotherm  of  a  would  have  arrived 
at  A,  &  at  B,  e  at  &,  and  so  on.  That  is  to  say,  the  lines 
marked  50°,  49°,  48°  would  be  in  the  same  places,  but 
would  be  numbered  47°,  46°,  45°  respectively,  though  the 
shape  of  the  contortions  would  be  the  same. 

If,  on  the  contrary,  the  direction  of  the  diurnal  slope 
had  changed  during  these  two  hours,  from  north-east  to 
north-west — that  is,  from  an  afternoon  to  a  morning 
aspect — then  the  shape  of  the  contorted  isotherms  would 
have  been  much  modified. 

This  conception  of  the  propagation  of  a  kind  of  diurnal 
wave,  and  the  superposition  of  cyclonic  or  anticyclonic 
heat-disturbance  on  its  slopes,  explains  most  satisfactorily 
what  the  author  has  so  often  observed  in  the  United 


210 


WEATHER. 


States  tri-daily  weather-maps,  viz.  that  the  shape  of  the 
isotherms  always  appears  to  change  more  between  the 
morning  and  afternoon  than  between  the  afternoon  and 
night  charts;  and  also  that  between  the  two  latter,  the  shape 
often  remained  pretty  constant,  though  the  numbering 
had  changed.  For  instance,  in  Figs.  42,  43,  44  we  give 


FIG.  42. — Diurnal  and  cyclone  temperature  (United  States). 

reductions  of  the  United  States  charts  at  11  p.m.  on  the 
22nd  of  January,  1873,  as  well  as  those  at  4.35  p.m.  and 
11  p.m.  the  following  day.  These  are  to  serve  a  twofold 
purpose — first,  to  show  why  the  distribution  of  temperature 
was  so  different  on  two  consecutive  days  at  the  same  hour, 


HEAT  AND   COLD. 


211 


viz.  11  p.m. ;  and,  secondly,  to  illustrate  the  diurnal  varia- 
tion in  the  shape  of  the  isotherms  between  4.35  p.m.  and 
11  p.m.  the  second  day. 

We  will  consider  the  latter  first.  The  isotherms  which 
we  see  on  the  4.35  p.m.  chart  (Fig.  43)  represent  the  dis- 
tribution of  temperature  due  to  the  influence  of  a  cyclone 


FIG.  43. — Diurnal  and  cyclone  temperature  (United  States). 

on  a  general  irregular  thermal  slope  from  the  equator  to 
the  pole,  as  modified  by  the  diurnal  range  of  the  season. 
The  aspect  of  the  diurnal  gradient  is  towards  the  north- 
east, because  the  temperature  is  falling. 

By  11  p.m.  the  same  day  (Fig.  44)  the  centre  of  the 


212 


WEATHER. 


cyclone  has  scarcely  moved,  and  the  general  shape  of  the 
isotherms  is  also  nearly  identical;  but  the  position  of 
the  isotherms  of  40°,  50°,  60°  at  4.35  p.m.  is  taken  broadly 
by  those  of  30°,  40°,  50°  at  11  p.m.,  and  the  place  of  10°, 
20°,  30°  at  4.35  is  less  nearly  approached  by  those  of 


FIG.  44. — Showing  diurnal   aud   cyclonic   temperature   in   the   United 

States. 

0°,  10°,  20°  at  11  p.m.  in  the  west.  This  means  that  the 
diurnal  range  was  less  in  the  north-west  than  in  the  south. 
The  interpretation  of  this  is,  that  the  aspect  of  the 
thermal  gradients  has  not  materially  changed,  though 
the  temperature  has  fallen  generally  nearly  10° ;  so  that 


HEAT  AND   COLD.  213 

the  shape  and  position  of  the  disturbance  of  temperature 
set  up  by  the  cyclone  remains  the  same,  but  the  number- 
ing of  the  isotherms  is  changed  nearly  10°. 


TEMPERATURE-DISTURBANCE  OF  A  CYCLONE. 

Now  that  we  have  eliminated  the  influence  of  diurnal 
range,  we  can  better  understand  the  nature  of  the  heat 
developed  by  a  cyclone.  In  the  same  three  figures  we 
have  got  rid  of  diurnal  range  by  two  methods.  By  taking 
the  charts  at  the  same  hour — 11  p.m. — in  the  first  and 
third  (Figs.  42  and  44),  diurnal  range  is  allowed  for  by 
being  equalized,  so  that  the  whole  of  the  difference 
between  these  two  sets  of  isotherms  is  due  to  general 
changes,  not  to  diurnal  variations. 

Then,  by  our  second  method  of  inferring  the  influence 
of  diurnal  slope  on  any  shape  of  isotherms,  we  are  enabled 
to  use  a  chart  at  the  intermediate  hour  of  4.45  p.m. 
(Fig.  43)  for  the  same  purpose  of  discovering  the  nature 
of  cyclone-heat. 

In  all  these  charts  we  see  that  the  general  nature  of 
the  development  of  heat  by  a  cyclone  consists  of  a  certain 
wedge-shaped  projection  of  the  isotherms  northwards  in 
front  and  on  the  southern  side  of  the  cyclone-centre,  and 
that  this  heat  moves  on  along  with  the  cyclone.  Observe 
that  the  local  seasonal  thermal  gradient,  from  the  cold 
interior  of  the  continent  to  the  warm  sea,  slopes  to  the 
north-west,  while  the  aspect  of  the  diurnal  thermal  slope 
is  towards  the  north-east  in  all  the  charts.  The  quality 
of  cyclone  heat  is  very  peculiar.  It  is  not  the  pleasant 
warmth  of  a  fine  day,  but  has  that  characteristic  close, 


214  WEATHER. 

inuggy,  disagreeable  feeling  which  we  have  before 
described  as  coining  before  cyclones.  This  is  the  kind 
of  heat  which  develops  neuralgia  and  similar  troubles 
in  old  wounds,  and  many  of  the  prognostics  which  are 
associated  with  the  front  of  a  cyclone.  We  could  not 
have  a  more  striking  instance  of  the  necessity  of  adding 
a  descriptive  account  to  all  instrumental  records  of 
weather.  Neither  a  thermogram  nor  a  synoptic  chart  can 
distinguish  between  one  kind  of  heat  and  another. 

The  cause  of  this  heat  is  obscure.  The  author  has 
shown  *  that  it  is  not  altogether  caused  by  that  backing 
of  the  wind  towards  the  south  which  precedes  the  rainy 
portion  of  a  depression,  and  that  the  rise  of  temperature 
seems  due  to  some  peculiar  property  of  cyclone-action. 

In  an  ordinary  whirl  of  dust  or  leaves  we  find  the 
particles  most  compressed  on  the  side  where  the  direc- 
tions of  rotation  and  translation  coincide ;  that  is  to 
say,  if  the  whirl  is  against  the  watch-hands,  and  the 
motion  in  any  direction,  the  compression  is  always  on  the 
right-hand  edge  of  the  eddy,  looking  towards  the  front. 

If  we  reflect  that  a  chart  of  cyclone-heat  shows  a 
wedge  projection  of  the  isotherms  on  a  general  thermal 
slope,  we  can  readily  understand  how  such  a  form  may  be 
analyzed  into  a  detached  patch  of  heat  lying  on  a  general 
thermal  slope.  We  are  thus  led  to  the  conception  of  a 
patch  of  heat  developed  by  the  cyclone,  and  moving  about 
with  it,  like  all  the  other  characteristics  of  such  a  whirl. 

For  this   reason,   we   often  find   exceptionally  high 

*  Abercromby,  "  On  the  Heat  and  Damp  which  accompany 
Cyclones,"  Quarterly  Journal  of  the  Meteorological  Society,  London, 
vol.  ii.  p.  274. 


HEAT  AND  COLD.  215 

temperature  on  the  north  of  a  cyclone-path  in  the  rare 
cases  when  the  propagation  of  the  depression  is  towards 
the  west  instead  of  towards  the  east,  as  is  generally  the 
rule.  We  shall  recur  to  the  importance  of  this  fact,  and 
give  an  illustration,  in  our  chapter  on  Forecasting  by 
Synoptic  Charts. 

But  we  may  now  describe  in  more  detail  the  tempera- 
ture-changes in  the  United  States  for  the  twenty-four 
hours  to  which  the  chart  refers. 

In  the  first  map  (Fig.  42)  the  centre  of  an  irregular 
cyclone  is  near  Memphis;  the  isotherm  of  50°  projects 
into  this  depression ;  the  isotherm  of  40°  reaches  nearly 
as  far  north  as  St.  Louis,  and  all  the  Mississippi  valley 
below  that  city  is  warm.  By  4.35  p.m.  the  next  day 
(Fig.  43)  the  cyclone  has  moved  in  a  north-west  direction 
to  Indianopolis,  and  the  isotherm  which  now  projects 
most  is  that  of  40°.  Temperature  has  fallen  all  over  the 
Mississippi  valley,  from  the  cold  winds  in  rear  of  the 
cyclone.  But  what  we  have  to  notice  most  are  the  tem- 
peratures recorded  at  the  Ohio  stations,  just,  in  front  of 
the  upward  projection  of  the  isotherm  of  40°,  which  were 
as  follows  -.—Toledo,  19°  ;  Cleveland,  27° ;  Pittsburgh,  31°. 
By  11  p.m.  the  same  evening  (Fig.  44)  the  centre  of  the 
cyclone  had  only  moved  a  few  miles,  but  that  was  suffi- 
cient to  bring  the  stations  just  mentioned  more  within 
the  range  of  higher  isotherms  than  earlier  in  the  after- 
noon. That  is  to  say,  the  thermometer  rose  at  all  those 
stations  between  4.35  p.m.  and  11  p.m.,  although  in  an 
ordinary  way  we  expect  to  see  the  mercury  fall  with  the 
sun.  The  actual  figures  were — Toledo,  3°,  Cleveland  and 
Pittsburgh  2°  each,  higher  than  in  the  afternoon. 


216  WEATHER. 

But  while  temperature  has  been  rising  in  Ohio,  many 
of  the  stations  in  the  lower  Mississippi  valley  have  lost 
from  5°  to  8°  from  diurnal  causes.  Other  stations,  such  as 
Montgomery,  Alabama,  have  lost  no  less  than  13°  from 
a  combination  of  diurnal  and  cyclonic  influences.  A 
glance  at  the  charts  will  enable  us  to  see  this  at  once,  for, 
while  the  Mississippi  stations  are  in  the  same  portion  of 
the  cyclone  at  both  hours,  the  latter  station  was  in  front 
of  the  cyclone  at  4.35  and  in  rear  at  11  p.m. 

A  few  years  ago,  no  explanation  could  have  been 
given  of  this  apparent  anomaly  of  the  air  getting  hotter 
as  the  sun  went  down ;  but  now  we  see  that  it  was  due  to 
the  temperature-disturbance  of  a  cyclone  overriding  the 
ordinary  variation  of  diurnal  influences.  In  our  chapter 
on  Meteograms  we  showed  how  similar  changes  would 
affect  the  trace  of  a  thermograph;  now,  to  complete  a 
comprehensive  view  of  the  subject,  we  have  illustrated 
the  same  phenomenon  by  the  totally  different  method  of 
synoptic  charts. 

Nothing  could  show  better  the  extreme  facility  with 
which  synoptic  charts  enable  us  to  study  temperature- 
changes  in  spite  of  diurnal  variation.  But  just  as  it  is 
not  at  all  obvious  at  first  sight  how  changes  in  the  position 
of  isobars  are  reflected  in  a  barogram,  so  nothing  but  a 
good  deal  of  experience  will  enable  the  meteorologist  to 
see  readily  how  changes  in  the  position  and  shape  of  the 
isotherms  would  affect  the  indications  of  a  single 
thermometer,  or  to  handle  with  any  ease  the  idea  of  the 
propagation  of  diurnal  isotherms  over  a  complicated 
system  of  temperature-distribution.  Our  example  is  one 
of  the  simplest  which  the  author  could  find.  In  the  first 


HEAT  AND   COLD.  217 

and  third  charts  we  equalize  diurnal  influences  by  con- 
structing the  maps  at  the  same  hour  each  day.  By  this 
means  we  can  explain  why  the  Atlantic  states  were  colder 
on  the  first  day  than  on  the  second,  and  why  the  Missis- 
sippi valley  was  colder  on  the  second  than  on  the  first  day. 

By  our  second  and  third  charts  we  illustrate  the 
manner  in  which  general  changes  override  diurnal  varia- 
tions when  the  latter  are  not  very  strong,  as  during  the 
winter  months,  as  well  as  the  characteristic  nature  of  a 
diurnal  fall  of  temperature  on  an  existing  system  of 
isotherms. 

There  are  nearly  eighty  stations  in  the  United  States 
and  Canada.  During  the  six  and  a  half  hours  in  question, 
changes  in  every  direction  of  varying  magnitude  took 
place  at  each ;  but  there  is  not  one,  however  apparently 
anomalous,  which  cannot  be  explained  by  means  of  the 
principles  which  we  have  here  laid  down. 

SOURCES  OF  HEAT. 

We  were  obliged  to  introduce  the  question  of  diurnal 
variation  of  temperature  in  the  first  place,  so  as  to  get 
rid  of  any  ideas  of  difficulty  from  that  source  of  com- 
plication ;  but  now,  before  we  describe  further  the  changes 
in  the  isotherms  from  day  to  day,  we  must  consider  the 
various  sources  of  heat  and  cold  with  which  we  have  to 
deal.  In  all  this  we  must  never  forget  that  the  natural 
distribution  of  temperature  is  an  irregular  thermal  slope 
from  the  equator  to  the  pole,  and  that  what  we  have  to 
explain  are  the  divergences  from  that  ideal  distribution 
which  we  find  in  practice.  We  shall  find  that  places 


218  WEATHER. 

far  north  are  sometimes  much  warmer  than  others  nearer 
the  equator,  and  that  some  parts  of  Europe  are  often 
colder  in  March  than  in  January.  All  these  apparent 
anomalies  we  can  explain  easily,  but  we  must  begin  with 
sources  of  heat. 

The  primary  source  of  heat  is,  of  course,  the  sun,  so 
that,  other  things  being  equal,  we  should  get  the  greatest 
heats  where  there  is  the  least  cloud ;  that  is  to  say, 
generally  in  anticyclones.  This,  however,  cannot  be  laid 
down  as  a  general  rule  without  some  modifications.  In 
the  belt  of  anticyclones  which  surround  the  world  about 
the  line  of  the  tropics,  some  of  the  greatest  known  heats 
are  recorded,  notably  in  the  Sahara  and  in  Australia. 
But  in  higher  latitudes  the  sun  has  a  powerful  enemy  in 
the  cold  space  which  surrounds  the  earth.  In  summer 
the  sun  is  the  more  powerful,  and  we  get  hot  days  with 
cold  nights.  In  winter-time,  when  the  sun  is  low,  radiation 
into  space  overpowers  the  radiation  from  a  low  sun,  and 
clear  weather  is  cold.  When,  then,  we  come  to  discuss  in 
general  terms  the  influence  of  cloud  on  isotherms,  we  must 
always  take  into  consideration  the  time  of  year  and 
latitude. 

Another  very  powerful  source  of  irregular  isotherms  is 
found  in  wind.  Of  course,  speaking  broadly,  southerly 
winds  will  deflect  the  isotherms  northwards,  and  northerly 
or  easterly  winds  will  bend  them  towards  the  south. 
This  too  is,  however,  subject  to  many  irregularities.  The 
great  difference  in  the  radiating  power  of  land  and  water 
at  different  seasons,  makes  a  continental  area  colder  in 
winter  and  hotter  in  summer  than  a  sea  in  the  same 
latitude.  For  this  reason  an  easterly  wind  from  a  land- 


HEAT  AND   COLD.  219 

area  would  blow  warm  into  a  neighbouring  sea  in  summer, 
and  cold  in  winter. 

We  have  already  alluded  to  the  idea  of  a  specific 
quality  of  heat  which  is  developed  by  cyclones,  and  minor 
local  sources  of  variation,  such  as  the  descending  winds 
which  probably  constitute  the  "fohn,"  need  only  be 
mentioned  here. 

When  all  these  sources  of  heat  are  combined,  a  vertical 
sun,  a  cloudless  sky,  a  southerly  wind,  and  an  arid  soil ; 
when  light,  hot  puffs  fill  the  air  with  scorched  particles  of 
sand,  till  the  dulled  sun  appears  to  glow  in  a  sea  of 
molten  brass,  and  the  poisoned  breath  of  the  simoon 
sweeps  fitfully  across  the  desert,  then  the  traveller  may 
well  beware,  and  hasten  for  his  life  to  the  nearest  shelter. 

An  example  of  great  heat  will  be  found  in  the  charts, 
Figs.  82  and  83,  which  illustrate  the  first  burst  of  the 
south-west  monsoon  in  Hindostan.  The  dates,  June  17 
and  18,  1881,  would  coincide  with  the  end  of  the  hot 
season  in  Northern  India.  Our  diagrams  show  on  both 
maps  a  patch  of  heat  over  100°  Fahr.  (38°  C.)  over  the 
Desert  of  Scinde ;  and,  as  this  would  be  at  about  half-past 
five  in  the  afternoon  locally,  it  is  certain  that  much  higher 
temperatures  must  have  been  recorded  nearer  midday. 
The  isobars  show  that  what  wind  there  was  would  be 
southerly  or  south-westerly,  and  of  course  light  from  the 
absence  of  gradient  near  the  centre  of  the  depression. 
The  soil  there  is  saltish  sand,  and  similar  material  has 
been  known  to  get  heated  up  to  nearly  200°  in  Australia. 
In  Scinde  there  is  a  dangerous  wind  at  this  season  exactly 
analogous  to  the  simoon  of  Arabia  and  the  Sahara.  Both 
are  certainly  allied  to  whirlwinds  and  tornadoes;  but, 


220  WEATHEE. 

unfortunately,  no  scientific  observations  have  been  made 
on  the  reputed  poisonous  or  fatal  character  of  these  blasts, 
or  of  the  dangerous  quality  of  heat  which  they  develop. 

There  is  one  type  of  warm  weather  in  Europe  for 
which  no  explanation  can  be  given  at  present.  We  have 
seen  that  an  anticyclone  usually  develops  cold- radiation 
weather,  but  sometimes  we  find  an  anticyclone  with  warm 
air  and  a  peculiar  soft  cloudy  sky.  This  anticyclone 
covers  Continental  Europe,  and  is  always  associated  with 
the  eastward  passage  of  distant  cyclones  on  the  northern 
side.  No  reason  can  be  assigned  for  this  heat;  all  we 
can  do  is  to  note  the  fact  for  future  research. 


SOURCES  OF  COLD. 

The  principal  source  of  all  cold  is  radiation  into 
space.  The  space  which  surrounds  the  earth  has  a 
theoretical  temperature  of  at  least  226°  below  0°  Fahr., 
and  it  is  the  influence  of  this  chilly  envelope  which  we 
feel. 

The  greater  part  of  the  influence  is,  however,  indirect. 
We  do  not  feel  the  cold  of  space  as  if  we  were  standing 
near  an  iceberg,  for  all  our  greatest  colds  are  produced  by 
radiation.  Bodies  on  the  earth's  surface  radiate  into  this 
cold  space  till  they  lose  a  large  amount  of  their  original 
temperature ;  and  air,  which  is  a  bad  radiator  itself,  gets 
cold  by  contact  with  the  chilly  soil. 

For  instance,  on  a  calm  winter  night  different  bodies — 
say,  a  sheet  of  iron  lying  on  the  ground  and  a  patch  of 
grass — begin  to  radiate  into  space  at  different  rates, 
according  to  their  own  intrinsic  properties.  Iron  radiates 


HEAT  AND  COLD.  221 

very  quickly,  but  is  also  such  a  good  conductor  that  it 
brings  up  an  abundant  supply  of  heat  from  the  ground  to 
replace  the  loss  by  radiation,  so  that  the  plate  does  not 
become  very  cold.  The  grass  is  a  less  good  radiator,  but 
at  the  same  time  a  very  bad  conductor ;  so,  though  it  parts 
with  heat  slower  than  the  iron,  it  cannot  replace  what  it 
has  lost  by  conduction,  and  therefore,  on  the  balance, 
becomes  much  colder.  This  is  the  cold  which  we  really 
feel,  and  which  sends  down  the  thermometer. 

A  very  striking  result  of  all  this  is,  that  under  these 
circumstances,  the  air  gets  warmer  as  we  ascend  up  to  a 
certain  height,  and  this  proves  conclusively  that  we  do 
not  feel  directly  the  chill  of  space.  Of  course,  the  greatest 
cold  will  be  produced  when  the  greatest  number  of  causes 
are  combined  which  favour  radiation.  These  are  a  still 
air,  a  clear  sky,  and  an  absence  of  water- vapour  in  any 
stratum  of  the  atmosphere.  This  last  condition  is  very 
interesting.  Professor  Tyndall's  researches  seem  to  show 
that  water-vapour  is  a  great  absorbent  of  the  quality  of 
heat  which  is  radiated  from  the  ground,  so  that  when 
much  vapour  is  present  the  ground  cannot  lose  its  heat 
so  rapidly  as  when  the  air  is  dry. 

All  these  conditions  of  great  cold  are  fulfilled  in  the 
most  perfect  manner  in  Siberia.  There  we  have  the 
centre  of  a  large  dry  continental  area,  which  in  winter- 
time is  persistently  covered  by  an  anticyclone ;  while  the 
latitude  is  so  high  that  the  sun  has  little  power.  Here, 
then,  we  find  calm,  dryness,  and  a  feeble  sun ;  and  here 
the  greatest  known  colds  are  reported,  if  we  except  some 
in  the  north  of  Smith's  Sound,  many  degrees  further 
north.  A  good  illustration  of  this  will  be  found  in  the 


222  WEATHER. 

two  charts  which  we  give  in  our  chapter  on  types  of  the 
north-east  monsoon  (Figs.  80  and  81).  In  them  we  see 
that  the  south  of  Siberia,  which  is  covered  by  an  anti- 
cyclone, has  stations  in  which  the  mercury  marks  more 
than  30°  below  zero,  Fahr. 

If  we  take  the  less  extreme  cases  which  occur  in 
Great  Britain,  we  find  that  all  frosts  in  that  country  are 
"home-brewed;"  that  is  to  say,  that  cold  winds  never 
bring  extremely  low  temperatures  from  the  plains  of 
Europe  or  the  mountains  of  Norway.  But  when  shallow 
gradients  for  east  and  north-east  winds  cover  Great 
Britain,  and  a  dry  chilly  air  favours  nocturnal  radiation, 
then  all  the  hardest  frosts  are  developed.  Then  we  often 
find  the  temperatures  10°  or  20°  lower  in  the  most  inland 
stations  of  England  and  Ireland,  and  the  isotherms 
gradually  increase  round  these  cold  centres.  When  we 
look  at  a  synoptic  chart  of  Europe  for  8  a.rn.,  we  find,  on 
these  occasions,  that  England  and  Ireland  are  separate 
islands  of  cold  on  the  general  thermal  slope  from  a  cold 
continent  to  the  warm  North  Atlantic.  From  the  fact 
that  frost  depends  on  radiation,  we  can  readily  explain 
why  cold  is  so  local.  Kadiation  is  very  sensitive  ;  the  least 
breath  of  wind  or  any  local  shelter  may  interfere  with 
the  free  play  of  radiation,  and  so  we  find  two  places  only 
a  few  miles  apart,  one  of  which  records  10°  or  15°  lower 
than  the  other. 

The  next  source  of  cold  is  found  in  wind.  When  this 
blows  from  a  frozen  continent,  then,  of  course,  very  low 
temperatures  may  be  recorded ;  but  this  is  not  the  same 
kind  of  cold  as  radiation-frost.  Here  we  have  another  of 
the  innumerable  instances  of  the  necessity  of  distinguish- 


HEAT  AND  COLD.  223 

ing  between  different  kinds  of  the  same  nominal  pheno- 
menon. The  1st  of  January  may  be  cold  in  one  year 
from  wind ;  in  another  from  radiation.  These  are  the 
products  of  totally  different  kinds  of  weather,  and  must 
not  be  mixed  up  in  scientific  meteorology. 

THE  "BLIZZARD"  AND  THE  "BARBER." 

A  very  striking  example  of  wind  is  found  in  the 
"  blizzards  "  of  the  United  States.  These  are  cold  snaps 
which  come  with  a  high  wind,  as  opposed  to  the  calm 
frost  of  anticyclones.  They  are  the  result  of  the  passage 
of  the  rear  of  cyclones  or  of  V-depressions  in  the  winter 
months,  such  as  we  see  in  Figs.  42  and  43.  Then  we  get 
high,  strong,  north-westerly  winds,  blowing  off  a  frozen 
continent  with  a  temperature  many  degrees  below  zero, 
and  with  surroundings  which  are  very  destructive  to  life. 
The  wind  drives  the  cold  into  the  bones  even  through 
fur  clothing,  and  raises  a  blinding  dust  of  powdery  snow. 
Under  these  circumstances  only  are  the  western  voyagers 
ever  lost.  If  wood  cannot  be  found,  nature  can  only  resist 
the  cold  for  a  certain  number  of  hours,  and  the  men  are 
frozen  to  death  if  no  shelter  can  be  reached.  A  very 
curious  circumstance  attends  these  deaths.  In  almost 
every  case  the  victims  are  found  to  have  begun  to  strip 
themselves.  When  the  body  is  nearly  reduced  to  an 
icicle,  only  a  very  little  blood  continues  to  circulate 
languidly  through  the  brain.  Then  delirium,  sets  in,  with 
a  delusive  sensation  of  heat,  under  the  influence  of 
which  the  traveller  begins  to  divest  himself  of  his  clothes. 

Another   disagreeable  form  of  cold  is  found  in  the 


224  WEATHER. 

St.  Lawrence  Gulf.  Sometimes  with  a  high  wind  the  air 
becomes  much  colder  than  the  open  water.  The  latter, 
being  relatively  hot,  begins  .to  smoke,  and  the  vapour 
freezes  into  peculiarly  sharp  spicules.  The  poudre  snow- 
crystals  of  the  north-west  are  usually  small,  dry,  six-sided 
petals,  and,  though  penetrating  as  sand,  they  are  soft.  The 
latter  kind  of  snow  is  so  damp  and  sharp  that,  when  driven 
by  a  gale,  it  nearly  cuts  the  skin  off  the  face.  Hence  the 
popular  name  of  the  "  barber,"  which  is  applied  to  this 
phenomenon.  The  same  name  of  "  barber  "  is  applied  to 
another  phase  of  cold  along  the  coasts  of  Nova  Scotia  and 
New  England.  When  a  vessel  is  caught  by  a  gale  of  wind 
in  a  cold  arctic  current,  the  spray  freezes  the  moment  it 
touches  the  deck  or  rigging.  Every  block  is  turned  into 
a  lump  of  ice ;  men  get  coated  with  ice  like  an  icicle ;  and 
sometimes  such  a  weight  of  ice  forms  on  the  bow  that 
the  stern  is  lifted  out  of  the  water,  and  the  ship  becomes 
unmanageable  for  want  of  steering  power. 

The  last  source  of  cold  which  we  need  mention  is 
rain.  All  rain,  of  course,  is  not  cold.  In  front  of  a 
cyclone  rain  is  warm,  and  a  shower  does  not  send  down 
the  thermometer.  In  the  rear,  on  the  contrary,  and  in 
thunderstorms  and  secondaries,  precipitation  is  more  or 
less  cold,  and  turns  the  mercury  downwards.  The  influence 
of  this  varies  very  much  in  different  countries  and  at 
different  seasons  of  the  year.  In  England,  during  the 
summer,  rainy  weather  is  cold,  because  it  cuts  off  the  sun, 
independent  of  any  chill  of  its  own.  In  winter,  on  the 
contrary,  rainy  weather  is  warm,  because  an  overcast  sky 
prevents  loss  of  heat  by  radiation.  In  the  tropics  cloudy 
weather  is  colder,  as  far  as  the  thermometer  is  concerned, 


HEAT  AND  COLD.  225 

than  a  bright  day,  because  the  rays  of  the  sun  are  ob- 
structed ;  but  if  there  is  little  wind,  a  cloudy  day  is  more 
oppressive  to  men  than  one  with  sunshine.  Near  the 
equator  there  is  very  little  diurnal  radiation  of  any  kind, 
owing  to  the  excessive  amount  of  vapour  in  the  air. 

We  may  sum  up  all  the  effects  of  heat  and  cold 
briefly  thus :  In  winter  wind,  cloud,  and  rain  in 
temperate  regions  tend  to  raise  the  temperature,  as  they 
check  cold  radiation ;  calm,  on  the  contrary,  induces  hard 
frost.  In  summer  wind,  cloud,  and  rain  are  cooling 
influences,  as  they  check  hot  radiation;  calm,  on  the 
contrary,  is  then  hot,  because  it  allows  full  play  for  the 
sun's  rays. 

We  may,  in  fact,  look  at  the  opposing  forces  of  hot 
and  cold  radiation  as  in  a  state  of  constant  conflict. 
The  rotundity  of  the  earth  always  weakens  the  power  of 
the  sun  in  the  north.  Water-vapour  in  some  shape 
forms,  as  it  were,  a  blanket  for  the  earth,  and  saves  her 
from  being  burnt  up  and  frozen  alternately.  The  incessant 
circulation  of  the  atmosphere  sometimes  eddies  in  a 
cyclonic  form,  and  develops  dense  cloud,  which  shields 
the  earth  from  the  radiation  of  the  season  and  latitude ; 
at  other  times  the  circulation  of  the  air  eddies  downwards 
in  an  anticyclone,  and  the  clear,  dry,  calm  atmosphere 
gives  full  play  to  radiation,  and  some  extreme  of  heat  or 
cold  is  then  developed. 

The  task  of  the  meteorologist  is  to  trace  how  the 
varying  forms  of  atmospheric  circulation  modify  the 
distribution  of  heat  and  cold  over  the  world  from  day  to 
day,  by  the  application  of  the  general  laws  we  have  just 
laid  down. 


226 


WEATHER. 


EXAMPLES  OF  DAILY  TEMPERATURE-CHANGES  OVER 
EUROPE. 

For  instance,  let  us  consider  the  changes  of  tempera- 
ture which  occurred  in  Europe  on  the  three  days, 
February  26-28,  1865 ;  that  is,  during  three  of  the  days 
for  which  we  shall  give  synoptic  charts  in  Figs.  68-70, 
when  discussing  the  westerly  type  of  weather.  We  have 
to  explain  now  why  European  temperature  varied  as  it 
did  on  those  days. 

The  isotherms  for  the  period  in  question  are  given  in 
Figs.  45-47.     In  all  of  these  there  is  a  thermal  gradient 


Feb.26. 1865, 8am.Green.wLch,.  Feb.23. 2866, 8cun.Breen.vnch. 

FIGS.  45  and  46. — Isotherms  in  Europe  for  three  consecutive  days. 

from  south-west  to  north-east  instead  of  a  slope  towards 
the  north-west,  as  undisturbed  natural  isotherms  should 
have  at  eight  o'clock  in  the  morning — the  hour  for  which 
the  charts  are  constructed.  The  reason  for  this  broad 
feature  is  that  in  winter  a  continental  area  is  always 


HEAT  AND  COLD. 


227 


colder  than  a  sea-surface,  and  therefore,  whatever  smaller 
variations  may  occur  from  day  to  day,  the  general  slope 
of  temperature  will  always  be  from  frozen  Russia  towards 
moisture-bathed  Portugal.  This  feature  belongs  to  the 
season,  and  is  found  in  every  chart ;  what  we  have  now 
to  explain  is  the  fluctuation 
in  the  position  of  the  iso- 
therms caused  by  the  vary- 
ing development  of  heat 
and  cold  locally. 

Glancing  at  both  the 
synoptic  charts  of  pressure 
and  temperature,  we  see 
that  on  the  morning  of 
February  26  a  Y-depression 
covered  Great  Britain,  with 

warm    SOUth-west    winds    in  Jieb.28.l865,8am.Greenwuh. 


FIG.  47. — Isotherms  in  Europe  for 
three  consecutive  days. 


front.  Straight  isobars  lay 
over  Scandinavia,  an  anti- 
cyclone stretched  over 

Western  Europe  from  the  Atlantic,  and  a  calm  col  lay  over 
Russia.  From  all  this  England  was  warm,  as  shown  by  the 
projection  northwards  of  the  isotherm,  of  41°  Fahr.  (5°  C.) ; 
Continental  Europe  and  Russia  were  very  cold.  In  the 
latter  country,  -4°  Fahr.  (-20°  C.)  is  reported,  and  local 
patches  of  cold  as  low  as  4°  Fahr.  (  —  15°  C.)  are  reported 
in  different  parts  of  France  and  Germany.  These  should 
be  noticed,  for  they  are  most  characteristic  of  the  abrupt 
local  variations  of  temperature  which  we  often  find  are 
caused  by  local  differences  in  radiation.  They  are 
identical  with  all  the  frosts  which  occur  in  Great  Britain, 


228  WEATHER. 

to  which  we  have  before  alluded.  Observe  also  that  they 
have  no  influence  whatever  as  a  cause  of  weather ;  they 
are  the  product  of  the  general  circulation  of  the  atmo- 
sphere, allowing  free  play  for  radiation,  not  a  cause  of 
that  circulation  themselves,  though  the  influence  of  the 
general  thermal  slope  from  Kussia  to  Portugal  is  an 
important  factor  in  determining  the  path  of  the  cyclones. 
By  next  morning  the  British  V  and  Scandinavian  straight 
isobars  have  formed  a  well-defined  cyclone,  some  second- 
aries appear  over  various  parts  of  Europe,  while  a  calm 
wedge  covers  England.  England  is,  therefore,  colder  than 
on  the  previous  day,  because  of  the  radiation  of  the  wedge, 
and  the  isotherm  of  41°  Fahr.  (5°  C.)  has  retreated 
southwards.  Russia  and  Continental  Europe  are  much 
warmer,  because  the  cyclones  and  secondaries  have 
destroyed  radiation. 

By  the  morning  of  the  third  day  the  Scandinavian 
cyclone  has  died  out,  but  a  new  one  lies  over  the  north 
of  Scotland.  Secondaries  still  cover  the  greater  portion 
of  Europe,  but  in  Eussia  the  weather  would  be  calmer. 
From  this  it  results  that  England  is  warmer,  so  that  the 
isotherm  of  41°  Fahr.  (5°  C.)  projects  northwards  again; 
Continental  Europe  is  a  little  colder,  \vithout  many  local 
frosts ;  Eussia  is  a  great  deal  colder,  but  not  so  cold  as 
the  first  day,  for  the  conditions  are  less  favourable  to 
radiation.  None  of  these  cyclonic  changes  reach  so  far 
south  as  Spain,  and  therefore  we  see  the  isotherm  of 
50°  Fahr.  (10°  C.)  scarcely  alters  its  position  during 
the  three  days. 

We  may  also  put  the  changes  of  temperature  over 
Europe  in  a  very  striking  light  by  looking  at  the  isotherm 


HEAT   AND  COLD.  229 

of  32°  Fahr.  (0°  C.).  On  the  first  day  it  stretches  from 
Belgium  to  the  Black  Sea ;  the  second  day  it  has  been 
driven  back  almost  to  the  Gulf  of  Bothnia  and  to  Poland  ; 
the  third  day  it  has  advanced  again,  but  not  so  far  as  on 
the  first  day.  So  on  the  conflict  would  go  between  the 
frost  and  sun  till  the  sun  at  last  drove  that  isotherm  out 
of  Europe.  In  the  autumn  the  battle  would  be  renewed ; 
but  then  the  sun  would  be  beaten,  and  frost  remain 
supreme  for  several  months  in  the  more  northern  portions 
of  that  continent. 

Had  our  limits  permitted,  we  would  have  given 
examples  of  the  reversal  of  radiation  effect  which  occurs 
in  summer,  when  an  anticyclone  means  heat  instead  of 
cold.  Then  we  may  often  find  England  hotter  than 
France,  for  if  the  calm  centre  of  an  anticyclone  lies  over 
the  former  country,  the  sun's  rays  have  more  power  there 
than  in  the  more  windy  southern  edge,  which  would  cover 
France  under  these  circumstances. 

We  may,  however,  refer  again  to  Figs.  21  and  22, 
which  relate  to  the  same  day  of  May — the  17th — in  two 
different  years,  and  in  which  diurnal  variation  is  allowed 
for  by  constructing  the  charts  at  the  same  hour.  On  the 
first  day  (Fig.  21)  a  cyclone  covers  Great  Britain,  and 
the  isotherm  of  50°  Fahr.  (10°  C.)  reaches  to  the  north 
of  Scotland  and  Denmark,  under  the  influence  of  southerly 
winds  and  a  cloudy  sky. 

On  the  second  day  (Fig.  22)  the  isotherm  of  50°  Fahr. 
(10°  C.)  runs  north  and  south  down  England,  and  a  corner 
of  the  line  of  40°  Fahr.  (5°  C.)  appears  over  Northern 
Germany.  This  shows  that  to  the  west  of  the  isotherm 
of  50°  the  temperature  rises  towards  60°  Fahr.  (15°  C.), 


230  WEATHER. 

and  therefore  that  part  of  England  and  Ireland  is  warmer 
than  on  the  same  day  of  another  year,  when  no  place 
recorded  anything  as  high  as  50°.  This  is  the  product 
of  the  calm  blue  sky  of  an  anticyclone ;  while  the 
diminished  temperature  over  Germany  is  due  to  the 
general  thermal  slope  of  the  season,  for  Continental 
Europe  does  not  get  warm  till  the  month  of  June. 

If  we  combine  all  these  with  the  other  examples 
we  have  already  given  of  temperature  ranging  from 
100°  Fahr.  (40°  C.)  to  -30°  Fahr.  (-35°  C.)  in  Europe, 
Asia,  and  America,  the  reader  will  have  a  very  fair  idea 
of  the  nature  of  temperature- changes. 


FORECASTING  TEMPERATURE. 

From  all  this  it  will  be  very  evident  that,  though  we 
can  lay  down  some  general  laws  of  temperature-changes, 
still  the  modifications  which  occur  in  practice  are  endless. 
The  forecaster  in  every  country  has  to  learn  by  experience 
the  qualities  of  the  different  winds,  and  the  power  of  the 
different  radiations  at  each  season  of  the  year. 

For  instance,  in  Great  Britain  he  soon  learns  to  dis- 
tinguish between  the  uniformly  warm,  close  heat  of 
winter  cyclones  ;  the  oppressive,  sultry  heat  of  a  summer 
thunderstorm ;  and  the  clear,  cold  air,  with  a  hot  sun,  of 
a  spring  anticyclone.  Any  doubt  which  can  arise  as  to 
the  future  course  of  temperature-changes  depends  on  the 
same  points  which  always  make  any  forecasting  uncertain 
— viz.  the  difficulty  of  knowing  what  the  future  path  of 
the  cyclones  will  be,  or  whether  any  new  distribution 
of  pressure  is  likely  to  set  in  suddenly.  If  the  forecaster 


HEAT  AND  COLD.  231 

judges  rightly  as  to  the  future  movements  of  pressure- 
distribution,  he  rarely  makes  a  mistake  as  to  the  nature 
of  temperature-changes  which  accompany  them. 

PRIMAKY  AND  SECONDARY  EFFECTS  OF  HEAT. 

We  will  conclude  with  one  important  reflection.  We 
know  that  heat  is  the  prime  mover  of  all  atmospheric 
circulation ;  why,  then,  do  the  great  local  differences  of 
temperature  have  so  little  influence  on  the  sequence  of 
weather?  The  greatest  diurnal  ranges  are  found  in 
anticyclones,  which  are  also  associated  with  the  steadiest 
weather ;  and  in  wedges,  where  we  find  strong  contrasts 
of  heat  and  cold,  these  local  differences  of  temperature 
are  certainly  not  the  cause  of  the  cyclone  and  rain  which 
follow  soon.  At  the  same  time,  it  is  certain  that  the 
persistent  anticyclone  over  Siberia  during  the  winter 
months  is  caused  by  the  radiation  cold  of  that  country. 
That  is  to  say,  we  may  conceive  that  in  the  general 
circulation  of  the  hot  air  of  the  equator  towards  the  pole, 
the  direction  of  the  currents  will  be  profoundly  modified 
by  the  surface-temperature  of  the  earth,  and  that  it  is 
perhaps  easier  to  flow  over  a  cold  surface  at  one  season 
and  a  warm  one  at  another. 

However  that  may  be,  we  are  met  by  the  apparent 
contradiction  that,  though  the  daily  variations  of  tempera- 
ture are  undoubtedly  the  product  of  the  motion  of 
cyclones,  etc.,  the  broad  situations  of  the  areas  of  cyclone 
activity  are  themselves  due  to  radiation. 

The  truth  probably  is  that  both  inferences  are  correct 
in  a  modified  degree,  and  that  in  this,  as  in  every  other 


232  WEATHER. 

meteorological  problem,  we  have  to  deal  with  a  balance 
of  influences  which  act  and  react  on  one  another  in  a 
very  complicated  manner. 

We  have  already  explained  the  stability  of  a  cir- 
culatory system  such  as  a  cyclone  or  anticyclone,  and 
the  idea  that  diurnal  variations  may  merely  affect  the 
rapidity,  but  not  the  form,  of  the  vortex  system ;  but 
one  observation  may  perhaps  be  noted  here  which  pro- 
bably has  some  bearing  on  the  question.  Our  synoptic 
charts  give  surface-temperature  only,  but  we  have  taken 
no  notice  of  the  heat  of  upper  currents.  Now,  it  has 
been  discovered  that  over  cyclones  temperature  diminishes 
from  the  surface  upwards  at  the  rate  of  about  3°  Fahr. 
(1*5°  C.)  in  one  thousand  feet ;  in  anticyclones,  on  the 
contrary,  when  radiation  produces  frost,  the  air  gets 
warmer  as  we  ascend  to  a  short  distance,  after  which  the 
temperature  begins  to  fall  as  we  go  higher  up. 

What  the  precise  significance  of  this  may  be  we 
cannot  tell,  but  it  is  interesting  in  connection  with  the 
manner  in  which  pressure  decreases  at  a  slower  rate  over 
cyclones  as  compared  with  anticyclones.  Further  research 
can  alone  solve  these  problems,  to  which  we  have  merely 
alluded  to  carry  out  our  purpose  of  giving  a  picture  of 
the  state  of  meteorology  at  the  present  day. 


(    2G3    ) 


CHAPTER  VIII. 
SQUALLS,  THUNDERSTOEMS,  AND  NON-ISOBARIC  RAINS. 

IN  this  chapter  we  propose  to  introduce  the  reader  to 
details  of  weather  totally  different  from  any  that  we  have 
hitherto  described.  So  far  we  have  dealt  with  the 
phenomena  of  wind  and  rain  which  are  associated  with 
cyclones  and  rapid  changes  of  barometric  pressure ;  now 
we  intend  to  discuss  changes  of  weather  which  are  con- 
nected but  indirectly  with  the  distribution  of  surrounding 
pressure,  and  in  which,  if  the  mercury  moves  at  all,  the 
direction  is  upwards.  Isobars  which  have  been  our 
unerring  guide  through  the  most  complicated  cyclonic 
weather  will  now  totally  fail  us ;  and,  under  the  heading 
of  non-isobaric  rains,  we  shall  discuss  certain  rainfalls,  to 
the  origin  of  which  we  have  at  present  but  little  clue. 
In  addition  to  the  interest  which  attaches  to  such  striking 
manifestations  of  nature  as  squalls,  thunderstorms,  tor- 
nadoes, and  whirlwinds,  a  great  deal  of  research  has  been 
bestowed  of  recent  years  on  these  subjects  which  has  not 
yet  found  its  way  into  popular  literature,  and  which  at 
present  is  scarcely  known  beyond  the  limited  circle  of 


234  WEATHER. 

professional  meteorologists.  We  now  propose  to  explain 
some  of  the  most  remarkable  results  which  have  thus 
been  obtained. 


SIMPLE  SQUALLS. 

If  we  watch  the  stages  of  gradually  increasing  wind, 
we  find  that  as  the  strength  rises  the  tendency  is  more 
and  more  to  blow  in  gusts.     Gradually  these  gusts  get 
still  more  violent,  and  in  their  highest  development  come 
with   a  boom   like  the  discharge  of  a  piece  of  heavy 
ordnance.     This  is  what  sailors  call  "  blowing  in   great 
guns,"  and  these  are  the  gusts   which  blow   sails   into 
ribbons,   and   dismast  ships  more   than  any  amount   of 
steady  wind.     These  gusts  only  last  a  few  minutes,  but 
they  seem  to  be  very  closely  allied  to  the  simplest  form 
of  squalls.     In  a  true,  simple  squall  the  wind  generally 
need  not  be  of  the  exceptional  violence  which   causes 
"  guns  ;  "  but  after  it  has  rather  fallen  a  little,  the  blast 
comes  on  suddenly  with  a  burst,  and  rain  or  hail,  accord- 
ing to  intensity,  or  other  circumstances,  while  the  whole 
rarely  lasts  more  than  five  or  ten  minutes.     At  sea  one 
often  sees  two  or  three  squalls  flying  about  at  a  time. 
Then  we  readily  observe  that  over  the  squall  there  is  firm, 
hard,  cumulus  cloud ;  that  the  disturbance  only  reaches 
a  short   distance   above   the   earth's   surface ;    that   the 
squall  moves  nearly  in  the  same  direction  as  the  wind ; 
and  that  there  is  little  or  no  shift  of  the  wind  before  or 
during  the  squall.     We  also  see  that  the  shape  of  the 
squall   is   merely   that   of    an   irregular   patch,   with   a 
tendency  rather  to  be  longer  in  the  direction  of  the  wind 


SQUALLS,  THUNDERSTORMS,   NON-ISOBARIC  RAINS.      235 

than  in  any  other  quarter ;  and  that  the  motion  of  the 
squall  as  a  whole  is  much  slower  than  that  of  the  wind 
which  accompanies  the  first  blasts.  If,  at  the  same  time, 
we  watch  our  barometer  closely,  we  find  that  if  the  squall 
is  sufficiently  strong,  the  mercury  invariably  rises — some- 
times as  much  as  one-tenth  of  an  inch — and  returns  to  its 
former  level  after  the  squall  is  over.  No  difference  is 
observed  in  this  sudden  rise,  whether  the  squall  is  accom- 
panied with  rain,  hail,  or  thunder  and  lightning;  and 
though  we  are  unable  exactly  to  explain  why  the  wind 
sometimes  takes  this  irregular  method  of  blowing,  we 
have  still  to  do  with  a  comparatively  simple  phenomenon. 

THUNDER-SQUALLS. 

The  simplest  kind  of  thunderstorm  may  more  properly 
be  described  as  a  squall  accompanied  by  thunder  and 
lightning,  instead  of  only  with  wind  and  rain.  In  Great 
Britain  these  thunder-squalls  are  very  common  on  our 
extreme  west  and  north-west  coasts  during  the  winter 
months,  while  they  are  very  rare  in  Central  or  Eastern 
England  at  any  season  of  the  year.  On  a  wild,  stormy 
day,  with  common  squalls,  one  or  two  of  these,  which  are 
exceptionally  violent,  will  be  accompanied  by  one  or  two 
claps  of  thunder  with  lightning.  The  principal  interest 
which  attaches  to  this  type  of  thunderstorm  consists  in 
the  proof  which  is  afforded  that  there  is  no  essential 
difference  between  a  common  squall  and  another  which 
may  be  associated  with  electrical  discharge,  except 
intensity.  The  look  and  motion  of  the  clouds,  and  the 
sudden  rise  of  the  barometer,  are  identical  in  both  cases. 


236  WEATHER. 

We  can  readily  conceive,  since  the  formation  of  cumulus 
above  the  squall  points  unmistakably  to  the  presence  of 
an  ascensional  current,  that  when  the  uptake  is  only 
moderate,  the  condensation  of  vapour  may  take  place  so 
gradually  that  none  of  the  electricity — which  there  is 
reason  to  believe  is  given  off  under  these  circumstances- 
is  discharged  disruptively ;  but  when  the  uprush  is  so 
violent  as  to  inject  the  moist  air  into  strata  which  are  so 
cold  and  dry  that  the  electricity  cannot  pass  off  silently, 
then  a  disruptive  discharge  with  thunder  and  lightning 
will  be  produced.  In  Western  Europe  this  class  of 
thunderstorm  is  much  more  common  in  winter  than  in 
summer,  which  is  the  reverse  of  what  takes  place  with  all 
other  kinds  of  thunderstorm.  So  much  is  this  the  case 
that  in  Iceland  there  are  no  summer  thunderstorm?,  but 
only  winter  ones,  of  this  simple  squall  type.  In  Norway 
both  types  occur  ;  and  the  winter  ones  are  there  found 
to  be  the  most  destructive,  because  they  are  lower 
down,  and  therefore  the  lightning  is  the  more  likely  to 
strike  buildings.  In  that  country,  however,  the  summer 
thunderstorms  are  not  nearly  so  violent  as  in  more 
southern  latitudes. 


BAROMETER  IN  SQUALLS  AND  THUNDERSTORMS. 

We  have  just  mentioned  that  the  barometer  usually 
rises  just  as  the  rain  of  a  squall  or  thunderstorm  strikes 
a  place,  and  this  is  as  true  on  the  Equator  as  on  the 
Arctic  Circle.  Since  this  fact  is  of  great  importance  in 
the  discussion  of  the  more  complicated  phenomena  that 
are  called  line-squalls,  we  will  devote  a  few  paragraphs  to 


SQUALLS;  THUNDERSTORMS,  NON-ISOBARIC  RAINS.    237 

the  elucidation  of  the  details  of  these  barometric  fluctua- 
tions.    We  can  do  this  best   RBBSSSH^^B9| 
by  an  actual  example. 

In  Fig.  48  we  give  a  pho- 
tographic engraving  of  the 
barometer-trace  given  by  the 
author's  barograph  in  Lon- 
don, on  May  18,  1878.  The 
original  was  recorded  on 
smoked  paper,  and  is  here 
reproduced  by  photography, 
absolutely  untouched  by  the 
engraver.  By  this  means 
the  most  delicate  fluctuations 
are  faithfully  rendered,  and 
those  who  are  familiar  with 
sensitive  self-recording  in- 
struments will  readily  recog- 
nize that  characteristic  un- 
easiness of  the  whole  trace, 
which  can  never  be  copied 
by  hand.  In  the  figure  the 
vertical  lines  represent  inter- 
vals of  six  hours,  while  the 
horizontal  lines  indicate  a 
difference  of  0'5  inch  of  mer- 
cury. Confining  our  atten- 
tion to  the  right-hand  portion 
only  of  the  diagram,  we  have 
to  note,  soon  after  midnight  of  May  17,  a  small  curious 
dip  of  the  barometer,  followed  immediately  by  a  sudden  rise. 


238  WEATHER. 

This  is  marked  a,  and  it  occurred  during  a  thunderstorm. 
Just  before  6  a.m.,  and  for  some  time  after,  we  find  the 
still  more  remarkable  fluctuations  marked  b.  These  were 
also  associated  with  a  series  of  thunderstorms,  none  of 
which  were  particularly  violent.  Still  later,  about  8  a.m., 
we  see  the  singular  dip  marked  c.  This  occurred  with 
gloomy,  threatening  weather,  but  neither  with  wind,  rain, 
nor  thunder,  at  the  place  of  observation  in  London. 

The  chart  for  the  day  at  8  a.m.  showed  that  a  series 
of  small  secondaries  lay  over  Great  Britain,  but  there 
were  no  bends  in  the  isobars  that  would  explain  such 
curious  barometric  oscillations. 

The  origin  of  this  characteristic  rise  of  the  barometer 
in  squalls  and  thunderstorms  is  at  present  unknown.  It 
has  been  suggested  that  it  is  due  to  a  rush  of  air,  carried 
down  by  the  rain.  That  such  is  partially  the  cause  is 
extremely  probable,  for  we  sometimes  see  a  small  rise 
under  a  heavy  splash  of  rain  without  either  thunder  or 
wind.  But  it  is  equally  certain  that  this  downrush  does 
not  entirely  explain  the  phenomenon,  for  sometimes  a 
rise  occurs  without  any  rain  at  all,  or  of  an  amount  which 
bears  no  relation  to  the  heaviness  of  the  fall.  Still  more 
puzzling  are  the  small  dips  of  the  mercury  which  we 
occasionally  find  with  thunderstorms,  and  of  which  some 
examples  are  given  in  our  last  figure.  These  dips  are 
more  rare  than  the  rises,  and  though  in  most  cases  they 
are,  as  in  this  example,  more  or  less  associated  with  the 
rises,  still  they  occasionally  occur  alone.  In  the  first  dip 
shown  in  our  last  figure,  about  1  a.m.,  the  depression  was 
associated  with  a  storm ;  while  in  the  second  case,  about 
8  a.m.,  no  storm  or  rain  occurred  locally,  though  un- 


SQUALLS,   THUNDERSTORMS,  NON-ISOBARIC   RAINS.       239 

doubtedly  storms  were  in  existence  not  far  off.  We  are, 
therefore,  almost  driven  to  the  conclusion  that  some  of 
these  curious  fluctuations  of  the  barometer  must  be  due 
to  a  sort  of  true  wave-action,  through  which  the  disturb- 
ance, caused  perhaps  by  falling  rain,  may  be  propagated 
by  the  elasticity  of  the  air  to  some  distance  from  the 
place  of  original  disturbance.  In  connection  with  this 
idea  of  air  being  brought  down  by  falling  rain,  we  may 
notice  that  very  striking  effects  are  sometimes  observed 
in  avalanches  of  snow,  which  always  bring  down  an 
immense  amount  of  imprisoned  air  with  them.  It  is 
usually  found  that  persons  caught  in  the  blast  of  the 
avalanche  have  their  clothes  torn  into  ribbons.  The 
suggestion  has  also  been  made  that  if  rain  is  the  product 
of  the  condensation  of  an  ascensional  current  of  air,  then 
the  more  violent  the  uptake,  the  greater  must  be  the 
reaction  downwards;  but,  unfortunately,  our  knowledge  of 
the  dynamics  of  air  in  motion  is  not  sufficiently  advanced 
to  enable  us  to  say  exactly  what  the  nature  of  pressure 
would  be  under  these  circumstances. 

But  though  we  cannot  altogether  explain  the  origin 
of  these  barometric  fluctuations,  we  know  enough  to  say 
that  they  are  of  a  totally  different  nature  from  any 
motion  of  the  mercurial  column  due  to  the  action  of 
cyclones  or  the  propagation  of  isobars  over  any  station. 
When,  then,  we  see  on  a  barogram  these  peculiar 
irregularities,  we  can  at  once  infer  that  they  are  the 
product  of  squalls  or  thunderstorms,  and  not  of  cyclones, 
and  so  far  we  are  enabled  to  increase  our  knowledge  of 
the  method  of  reading  barograms,  to  which  we  have 
already  given  so  much  attention.  These  dips  and  rises 


240  WEATHER. 

may,  in  fact,  be  taken  as  another  letter  in  that  barographic 
alphabet  by  which  a  skilled  meteorologist  can  read  the 
history  of  the  weather  from  his  barometric  trace.  Another 
very  important  inference  which  this  knowledge  gives  us 
is  that,  as  these  rises  are  entirely  different  from  those 
due  to  cyclonic  motions,  we  cannot  make  the  same  deduc- 
tions from  the  one  that  we  would  from  the  other.  For 
instance,  if  in  rear  of  a  cyclone  the  mercury  rose  at  the 
rate  of  two-tenths  of  an  inch  in  an  hour,  that  would  be  an 
exceptional  rate  anywhere  in  Europe,  and  we  should  expect 
that  it  would  be  associated  with  a  violent  gale.  But  the 
mercury  might  rise  at  two  or  three  times  that  rate  in  a 
thunderstorm  of  no  exceptional  intensity,  with  which  there 
would  be  no  more  than  a  few  irregular  gusts.  For  the 
same  reason,  if  we  have  to  include  any  barometric  readings 
of  these  peculiar  rises  in  the  construction  of  our  synoptic 
charts,  we  must  not  draw  the  same  conclusions  from  the 
lie  of  the  isobars  as  we  should  in  ordinary  weather,  be- 
cause the  origin  of  the  isobars  is  not  the  same  in  both 
cases.  The  error  which  we  have  to  avoid  is  not  to  take 
as  the  same  two  phenomena  that  are  really  totally  distinct, 
but  which  have  one  common  property  — namely,  a  rise  of 
the  barometer. 


LINE-SQUALLS. 

We  have  already  explained  that  the  line  of  the  trough 
of  a  cyclone  or  V-depression  is  associated  with  a  line  of 
squalls,  and  that  we  must  picture  to  ourselves  a  long, 
narrow,  thin  band  of  rain  and  wind  sweeping  across  the 
country,  broadside  on,  like  a  wall  or  curtain,  at  the  same 


SQUALLS,   THUNDERSTORMS,  NON-ISOBARIC  RAINS.      241 

speed  as  the  depression  itself.  This  rate  bears  no  relation 
to  the  velocity  of  the  wind  in  the  squall.  In  practice  the 
rate  of  the  depression  will  be  much  slower  than  the  wind 
in  the  first  gust.  The  former  will  probably  not  exceed 
forty  miles  an  hour ;  the  latter  may  mount  up  to  seventy 
or  eighty  miles  an  hour.  We  will  now  go  into  some  very 
interesting  details  of  this  class  of  atmospheric  disturbance, 
which,  for  the  sake  of  classification,  we  will  call  "  Line- 
squalls." 

The  nature  of  this  class  of  squall  will  be  best  ex- 
plained by  an  actual  example  of  the  squall  which  capsized 
an  English  man-of-war — the  Eurydice — and  caused  one  of 
the  greatest  disasters  which  has  befallen  the  British  navy 
for  many  years.  In  Fig.  74,  under  weather- types,  we  give 
a  chart  of  a  large  portion  of  the  northern  hemisphere  for 
March  24,  1878,  at  0.43  p.m.,  Greenwich,  and  we  may 
just  glance  at  it  now  to  see  the  general  distribution  oi 
pressure  over  Europe  on  that  day.  The  squall  which  we 
have  now  to  consider  belonged  to  one  of  the  numerous 
secondaries,  which  hardly  show  on  the  large  chart ;  but  in 
Fig.  49  we  give  the  details  of  pressure,  wind,  and  weather 
over  Great  Britain  and  France  at  the  same  hour  on  a 
larger  scale.  In  this  diagram  we  see  an  extremely  com- 
plex distribution  of  pressure.  What  concerns  us  most  is 
the  bend  in  the  isobars,  along  which  we  have  run  a  dotted 
line  that  is  marked  "  trough  "  at  one  end.  This  bend  is 
a  small  V-depression,  in  some  way  secondary  to  the  ill- 
defined  fragment  of  a  cyclone  that  covers  the  southern 
portion  of  the  Scandinavian  peninsula.  During  the 
course  of  the  day  this  cyclone  appeared  to  circle  round 
another  which  lay  in  the  morning  over  the  Carpathian 


242 


WEATHER. 


Mountains ;  and,  in  connection  with  these  greater  changes, 
the  trough  of  the  V  wheeled  round  a  point  near  the  Scaw, 
in  Denmark,  like  the  spoke  of  a  wheel.  Fig.  49  shows 
the  position  of  the  trough  at  0.43  p.m. ;  the  front  line  of 
the  crescent-shaped  shaded  area  in  Fig.  50  shows  approxi- 
mately the  position  of  the  trough  at  3  p.m.;  and  by  6  p.m. 


FIG.   49. — The    Eurydice   squall. 
Isobars  and  wind  at  0.43  p.m. 


FIG.   50. — The     Eurydice     squall. 
Area  covered  by  squall  at  3  p.m. 


the  trough  passed  in  a  curved  line  from  Yarmouth,  through 
the  Straits  of  Dover,  into  Normandy.  By  reason  of  this 
wheeling  motion,  different  portions  of  the  trough  moved 
with  very  different  velocities.  Between  the  hours  just 
named,  the  northern  portion  of  the  trough  moved  across 
England  at  the  rate  of  only  thirteen  miles  an  hour,  while 
the  extreme  south-westerly  edge  traversed  the  country  at 


SQUALLS,  THUNDERSTORMS,  NON-ISOBARIC  RAINS.      243 

the  rate  of  no  less  than  forty- eight  miles  an  hour.  The 
portion  which  struck  the  Eurydice  was  going  at  the  rate 
of  thirty-eight  miles  an  hour. 

So  far  for  the  motion  of  the  V  as  a  whole.  In  Fig.  49 
the  wind  was  from  about  west  in  front,  and  from  north-west 
in  rear  of  the  V,  but  no  well-defined  area  of  rain  was  then 
developed.  By  3  p.m.,  however,  the  depression  had  so 
much  increased  its  intensity  that  Mr.  Ley  was  able  to 
•construct  the  diagram  given  in  Fig.  50.  In  that  figure 
the  shaded  portion  shows  the  area  over  which  rain  or 
snow  was  falling  at  the  moment ;  the  solid  arrows  give 
the  general  sweep  of  the  surface- winds,  the  dotted  ones 
those  of  the  upper  currents.  The  author  has  further  shown 
that  the  front  of  the  rain-area  was  essentially  coincident 
with  the  trough  of  the  V,  which  we  see  about  three  hours 
earlier  in  Fig.  49,  so  that  we  evidently  have  to  deal  with 
a  V  of  that  class  in  which  the  rain  is  in  rear  of  the 
trough.  At  every  station,  after  the  wind  had  been  from 
the  west,  with  a  cloudy  sky  in  the  morning,  the  clouds 
gradually  banked  up  ominously  to  the  north-west ;  then 
rain  or  snow  came  on  with  a  tremendous  squall,  while 
the  wind  jumped  round  to  north-west.  After  the  first 
burst  had  moderated,  rain  or  snow  continued  for  a  longer 
or  shorter  interval  till  the  sky  cleared  again. 

H.M.S.  Eurydice  was  a  full-rigged  corvette,  homeward 
bound  from  the  West  Indies.  At  3.45  p.m. — three- 
quarters  of  an  hour  later  than  that  for  which  the  chart 
given  in  Fig.  50  was  constructed — she  was  off  Ventnor,  in 
the  Isle  of  Wight,  running  free  before  a  nearly  westerly 
wind,  with  all  sail  set.  At  that  moment  she  was  struck 
by  a  squall  from  the  north-west;  before  sail  could  be 


244  WEATHER. 

shortened,  she  went  on  to  her  beam-ends,  and,  as  the  lee 
ports  were  open,  she  filled  and  foundered. 

On  the  whole  we  have,  therefore,  to  idealize  a  band- 
shaped  area  of  rain,  bounded  in  front  by  a  line  of  squalls 
— in  this  case  more  than  four  hundred  miles  long — sweep- 
ing broadside  on  across  Great  Britain  at  a  rate  varying 
from  thirteen  to  nearly  fifty  miles  an  hour.  From  this 
we  can  readily  see  how  places  many  miles  apart  can  be 
struck  simultaneously  at  the  same  hour,  and  how  ap- 
plicable is  the  name  of  line-squalls,  which  we  have  applied 
to  this  class  of  disturbance. 

Though  this  class  of  line-squall  is  uncommon  in 
Great  Britain,  it  appears  to  be  very  frequent  in  other  parts 
of  the  world.  For  instance,  in  Iowa,  a  similar  kind  of 
squall  is  peculiarly  characteristic  of  summer  weather. 
There  it  generally  occurs  after  a  spell  of  continued  hot, 
rather  sultry  weather,  the  wind  having  blown  steadily 
but  moderately  from  the  south  or  south-west,  the 
barometer  not  changing  much.  In  the  north-west  the 
storm-front  will  make  its  appearance ;  threatening,  dark, 
towering  clouds,  or  at  times  an  immense  roll-like  cloud, 
will  approach ;  the  air  cools  rapidly  as  the  storm-front 
comes  nearer ;  and,  with  a  high,  straight  blow,  bending 
young  trees  to  the  ground  and  driving  the  rain  nearly 
level,  the  fierce  storm  passes  over,  while  the  barometer 
rises  rapidly.  Such  a  blow  does  not  last  long,  but  may 
be  repeated  with  gradually  weakened  force  at  intervals. 
A  steady  pouring  rain  generally  follows,  after  which  the 
sky  clears,  and  the  storm-wind  wheels  back  to  the  south- 
east, the  weather  being  as  hot  as  before  the  storm.  This 
description,  which  we  have  taken  from  Dr.  Hinrichs,  of 


SQUALLS,  THUNDERSTORMS,   NON-ISOBARIC  RAINS.      245 

Iowa  City,  together  with,  the  maps  which  he  gives  to 
illustrate  them,  point  very  clearly  to  line-squalls,  asso- 
ciated with  that  class  of  V-depression  in  which  the  rain 
follows  the  trough.  His  maps  do  not  exhibit  the 
shape  of  the  rain-area  like  the  one  we  have  just  given, 
but  they  show  the  squalls  sweeping  across  the  State 
with  a  crescent-shaped  front  exactly  like  the  Eurydice 
squall. 

When  we  compare  this  class  of  squall  with  the  pure 
and  simple  squall  which  we  first  described,  it  will  be 
obvious  that  the  two  kinds  have  little  in  common  except 
the  name.  The  former  kind  seems  to  be  simply  a  local 
intensification  of  a  general  sweep  of  wind.  The  latter,  on 
the  contrary,  is  associated  with  a  very  definite  but  com- 
plex phase  of  aerial  circulation,  which  we  shall  understand 
better  when  we  have  described  a  precisely  analogous  class 
of  thunderstorms. 


THUNDERSTORMS  ASSOCIATED  WITH  LINE- SQUALLS. 

Squalls  are  rarely  of  sufficient  importance  to  attract 
the  notice  of  enough  observers  to  enable  the  details  of 
their  shape  and  progress  to  be  properly  determined  ;  but 
thunderstorms  are  such  a  striking  manifestation  of  weather 
that  they  are  much  more  easily  traced,  and  an  enormous 
amount  of  work  has  been  done  in  late  years  in  marking 
out  the  hourly  advances  and  development  of  such  dis- 
turbances. 

Many,  but  not  all,  European  thunderstorms  have  been 
found  to  be  precisely  similar  to  the  line-squall  which 
we  have  just  described.  Some  of  Bezold's  diagrams  of 


246  WEATHER. 

Bavarian  thunderstorms,  which  give  the  shape  of  the 
area  covered  by  the  storm  at  successive  hours,  show  long 
narrow  bands  sweeping  broadside  on  across  the  country 
exactly  analogous  to  the  squall-area  which  we  drew  in 
Fig.  50. 

But  the  remarkable  point  is,  that  though  some  of  these 
storm-bands  are  associated  with  the  troughs  of  cyclones 
and  V's,  precisely  similar  bands  are  more  often  found 
either  in  front  or  in  rear  of  the  cyclone,  where  we  can 
connect  them  (the  bands)  with  no  particular  part  of  the 
cyclone,  except  that  the  front  of  the  band  is  usually 
perpendicular  to  the  line  of  progress  of  the  depression  in 
which  it  is  formed. 

We  will  first  give  an  example  of  the  storm  and 
thunderstorm  associated  with  the  trough  of  a  V-depres- 
sion.  On  July  16, 1884,  at  about  6.15  p.m.,  the  trough  of 
a  V-depression  swept  over  Hamburg,  and  brought,  as 
usual,  a  violent  squall  and  heavy  rain,  with  much  thunder 
and  lightning.  This  was  only  a  section,  as  it  were,  of  a 
line-thunderstorm.  Dr.  Sprung,  by  combining  the  section 
of  barometric  and  other  curves  at  Hamburg  with  the 
records  of  other  observatories  and  the  synoptic  charts  of 
Germany,  at  8  p.m.  the  same  evening,  in  the  manner  that 
we  explained  in  our  chapter  on  Meteograms,  has  built  up 
the  beautiful  diagram  of  this  storm,  which  we  reproduce, 
with  a  few  trifling  modifications,  so  as  to  assimilate  it 
with  our  other  illustrations,  in  Fig.  51.  There  the  isobars 
are  given  both  in  millimetres  arid  their  approximate 
equivalents  in  inches :  t  marks  the  line  of  the  trough ; 
the  shaded  line  r  the  position  and  dimensions  of  the  rain- 
stripe  ;  the  long  dotted  arrow  the  direction  in  which  the 


SQUALLS,   THUNDERSTORMS,  NON-ISOBARIC   RAINS.      247 

whole  system  was  being  propagated ;  and  the  small  solid 
arrows  the  direction  and  force  of  the  wind  across  any 
section.  The  whole  is  evidently,  in  the  main,  one  of  those 
V's  in  which  the  rain  begins  just  after  the  passage  of  the 
trough  ;  but  the  curious  projection  upwards  of  the  isobars 
just  under  the  rain-stripe 
is  unlike  anything  we 
have  seen  before;  and 
the  strong  west  wind,  with 
four  feathers  on  the  arrow, 
is  quite  unconformable 
with  the  isobars  accord- 
ing to  our  usual  expe-  - 
rience.  From  whence 
comes  all  this?  First, 
for  the  upward  projec- 

,.  P  j.1        •     i  AH    FIG.  51. — Line-thunderstorm,  t,  Trough 

tlOn   Of  the  isobars.      All          of  ^depression ;  r,  rain  stripe. 

the  barographs    showed, 

about  seven  minutes  after  the  passage  of  the  trough,  a 
sudden  rise  exactly  similar  to  those  marked  b  in  Fig.  48 
just  as  the  heavy  rain  began,  and  this  rise,  as  usual,  was 
quite  distinct  from  the  general  increase  of  pressure  due  to 
the  rear  of  the  V.  If  we  were  to  superimpose  a  long, 
narrow,  isolated  ridge  of  high-pressure  on  the  rear  of 
a  V,  we  should  get  an  inverted  V,  or  wedge,  exactly  like 
that  which  we  find  in  the  diagram  under  the  rain-stripe ; 
but  we  must  not  treat  this  like  the  ordinary  wedge- 
shaped  isobars  which  we  have  before  described.  The 
form  is  the  same,  but  the  cause  is  different. 

Then  for  the  wind-sequence.     We  find  in  front  of  the 
V  a  light  south-east  wind  ;  then,  just  before  and  at  the 


248  WEATHER. 

commencement  of  the  rain,  a  very  violent  squall  (called 
"boe"  in  Germany)  from  the  west;  and,  finally,  a  light 
south-west  wind  in  rear  of  the  whole  disturbance.  The 
westerly  squall  is  nearly  perpendicular  to  the  isobars  and 
to  the  lie  of  the  rain-stripe ;  but,  as  the  isobars  here  are 
not  the  lines  of  general  atmospheric  circulation,  but  are 
partly  due  to  purely  local  causes,  we  need  not  be  surprised 
that  Buy  Ballot's  law  does  not  hold. 

Temperature  was,  as  usual,  very  high  in  front ;  very 
low  for  the  season  in  rear  of  the  trough.  If  we  could 
have  drawn  the  isotherms,  we  should  have  found  them 
running  almost  north  and  south,  nearly  parallel  to  the 
rain-stripe.  It  is  impossible  to  determine  at  present  how 
much  of  this  cold  is  due  to  the  mechanical  transport  of 
cold  air  with  the  heavy  rain,  and  how  much  to  the 
general  descent  of  cold  air  in  rear  of  the  V  as  a  whole. 

The  rain-stripe,  we  see,  was  in  this  instance  a  long 
narrow  band,  moving  broadside  on,  and  manifestly  con- 
nected with  the  trough  of  a  V-depression. 

We  will  now  give  an  example  of  line-thunderstorms 
which  are  not  associated  with  the  trough  of  either  a  V  or 
a  cyclone,  though  they  also  move  broadside  on,  nearly 
perpendicular  to  the  depression  with  which  they  are  in 
some  way  associated.  In  Fig.  52  we  give  synoptic  charts 
for  France  at  9  a.m.  and  9  p.m.  (Paris  time)  on  August  21, 
1879.  The  full  lines  are  isobars,  a  few  arrows  show  the 
general  direction  of  the  wind,  and  the  one  dotted  line  in 
each  marks  the  mean  position  of  the  thunderstorms  which 
were  raging  at  that  moment.  In  France  the  mean  of  the 
time  between  the  first  and  last  thunder  is  taken  to  give 
the  position  of  the  storm  at  a  given  hour.  For  instance, 


SQUALLS,  THUNDERSTORMS,  NON-ISOBARIC  BAINS.      249 

if  the  first  thunder  was  heard  in  a  place  at  8  a.m.  and  the 
last  at  10  a.m.,  then  9  a.m.  would  be  marked  as  the  hour 
at  which  the  storm  passed  the  station.  This  method  is 
manifestly  inferior  to  that  of  noting  the  times  of  first  and 
last  thunder,  and  then  plotting  the  shape  of  the  storm  on 
a  chart.  The  barometric  changes  during  the  day  were 


Aug.  2ii8^q.   q.p.m. 
FIG.  52. — Thunderstorms  in  France. 

really  much  more  complicated  than  might  appear  from 
an  inspection  of  the  maps  over  the  limited  area  of  France. 
The  large  secondary  cyclone  whose'  centre  lay  over  the 
west  of  France  in  the  morning,  appears  to  have  crossed 
that  country  in  a  north-easterly  direction,  and  to  have 
merged  in  a  complicated  manner  with  a  larger  depression 
which  lay  to  the  north-west  of  Ireland  in  the  early  morn- 
ing. By  9  p.m.,  however,  the  whole  of  the  south-west  of 
France  was  covered  by  an  anticyclone,  whose  origin  we 
cannot  trace. 

In  Fig.  53  we  give  a  diagram  of  the  positions  at  every 
alternate  hour  of  two  sets  of  thunderstorms  that  traversed 


250 


WEATHER. 


•if jr.  p.m. 


France  during  that  day,  which  are  marked  A  and  F 
respectively.  Such  lines  are  called  "  isobrontons,"  or 
lines  of  equal  development  of  thunder.  Those  in  the 
figure  may  be  taken  as  typical  of  the  march  of  this  class 
of  thunderstorm  all  over  Europe — a  long  narrow  line, 

advancing  broadside  on 
ir.p.m.  towards  the  east  or  north- 
east, pretty  nearly  indepen- 
dent of  the  shape  of  isobars 
with  which  it  is  associated. 

The  first  storm,  marked 
A,  struck  the  coasts  of  the 
Bay  of  Biscay  at  five  o'clock 
in  the  morning.  The  shape 
of  the  front  was  bent,  the 
ends  being  most  in  advance  : 
this  is  very  often  the  case  in 
France.  The  position  of  the  front  of  this  storm  is  givenTor 
every  two  hours'  interval  until  1  p.m.,  when  the  disturb- 
ance appears  to  have  died  out  near  Calais.  The  relation 
which  the  position  of  the  thunder  bore  to  the  secondary 
cyclone  may  best  be  seen  by  reference  to  the  preceding 
chart  (Fig.  52).  There  the  dotted  line  shows  that  the  front 
of  the  storm  A  lay  to  the  north-east  of  the  centre  of  the 
cyclone ;  and,  as  far  as  the  small  number  of  wind-arrows 
allow  us  to  judge  of  the  nature  of  the  disturbance,  there 
seems  to  be  a  small  local  deflection  of  the  wind  in  rear  of 
the  storm.  The  wind  in  rear  should  have  been  from 
south-east  with  the  shape  of  isobars  there  represented, 
whereas  the  chart  shows  that  in  some  places  the  direction 
was  from  the  west  and  north-west. 


FIG.  53. — Track   of   thunder- 
storms. 


SQUALLS,  THUNDERSTORMS,   NON-ISOBARIC  RAINS.      251 

The  second  series  of  thunderstorms,  marked  B,  com- 
menced at  Biarritz  at  three  o'clock  in  the  afternoon,  and 
moved  irregularly  in  a  more  westerly  direction  than  the 
morning  storm.  The  storm,  in  its  course  during  the  day, 
seems  to  have  increased  enormously,  for  at  9  p.m.  we  find 
the  front  reaching  nearly  from  Brussels  to  Perpignan  in 
the  Pyrenees,  a  distance  of  six  hundred  miles.  This  will 
perhaps  enable  us  to  realize  the  disastrous  character  of 
hail  and  thunderstorms  in  France.  Here  we  have  a  line  of 
destruction  six  hundred  miles  long,  and  from  ten  to  twenty 
miles  broad,  sweeping  like  a  curtain  across  the  country  at 
a  rate  of  about  thirty  miles  an  hour,  wrecking  in  a  few 
minutes  vineyards  which  are  worth  many  thousands  of 
pounds,  and  destroying  at  the  last  moment  the  husband- 
man's labour  for  the  whole  year. 

But  now  we  come  to  one  of  the  most  puzzling  points 
connected  with  thunderstorms.  If  we  look  at  Fig.  52, 
where  we  have  marked  by  a  dotted  line  the  position  of 
the  storm-front  B  at  9  p.m.,  it  is  very  difficult  to  see  any 
connection  between  the  shape  of  the  isobars  and  the 
position  of  the  thunder.  So  far  as  we  can  see,  there  is  no 
trace  of  the  trough  either  of  a  cyclone  or  of  a  V-depression, 
and  all  the  general  indications  would  have  been  for 
improving  weather  after  the  passage  of  the  secondary 
cyclone.  Whether  more  numerous  observations  at 
stations  nearer  to  one  another  would  have  shown  the 
presence  of  secondaries,  we  cannot  say ;  but  this  is  by  no 
means  an  isolated  instance  in  France,  and  similar  cases 
occur  in  other  countries.  For  the  present,  therefore,  the 
explanation  of  the  nature  of  this  class  of  thunderstorms 
must  await  future  research  ;  all  that  we  can  do  here  is  to 


252  WEATHER. 

note  them  as  an  apparent  exception  to  the  general  course 
of  weather  which  we  have  already  explained.  We  must 
also  specially  note  them  as  cases  where  rain  falls  with 
a  steady  rising  barometer;  and  also  understand  that, 
although  a  forecaster  could  not  have  pointed  out  in  the 
morning  exactly  when  or  where  thunderstorms  would 
-strike,  he  could  have  said  with  certainty  that  storms 
would  occur  during  the  day  in  many  parts  of  France. 
Secondaries  can  never  form  in  summer  without  some 
electrical  disturbance. 

The  details  of  rain  and  cloud  in  line-squalls  and 
thunderstorms  are  extremely  interesting,  and  for  them 
we  are  chiefly  indebted  to  the  careful  researches  of  Dr. 
Koppen.  The  approach  of  a  thunder-squall  usually 
announces  itself  by  the  rapid  crowding  up  of  heavy 
clouds.  We  see  a  dark,  black  border,  often  looking  like 
a  long  roll  or  wreath,  and  beyond  this  a  peculiar  light 
grey  uniform  sky.  The  dark,  low  band  passes  overhead, 
and  heavy  rain  commences  as  the  light  grey  cloud  comes 
on.  The  first  burst  of  rain  is  usually  the  heaviest,  and 
after  a  longer  or  shorter  period  the  rain  usually  clears 
gradually  off.  This  is  the  rain  with  which  the  sudden 
rise  of  the  barometer  is  observed.  The  wind,  which  has 
fallen  very  light  from  south-east  or  south  as  the  clouds 
begin  to  bank  up,  comes  in  a  violent  squall  from  the 
west,  about  the  time  the  dark  wreath  passes  overhead, 
and  falls  again  shortly  after  the  commencement  of  the 
heavy  rain.  A  good  illustration  of  a  very  pronounced 
cloud-wreath  will  be  found  in  Fig.  56  in  the  next  chapter 
under  "Pamperos."  We  give  an  ideal  diagrammatic 
section  of  such  a  squall  in  Fig.  54.  We  may  suppose 


SQUALLS,  THUNDERSTORMS,  NON-ISOBARIG   RAINS.      253 

that  from  general  causes,  such  as  the  trough  of  a  V-de- 
pression,  a  cold  westerly  current  meets  a  warmer  one 
from  the  south-east  or  south.  The  latter  rises,  as  shown 
by  the  small  arrows,  and  curls  over  where  the  black 
wreath  (w)  of  cloud  is  found,  and  then  the  commingling 
of  the  two  currents  forms  a  gigantic  dark  vault  (v)  of 
cloud,  from  which  heavy  rain  (r)  pours  down.  The  light 


FIG.  54. — General  circulation  and  cloud-vault  of  line-squall. 

grey  cloud  which  an  observer  sees  behind  the  black 
wreath  is  really  a  peep  into  the  rain  falling  from  this 
great  vault.  The  big  drops  of  rain  bring  down  mechani- 
cally with  them  a  vast  amount  of  cold  air,  which  rushes 
straight  out  in  front  of  the  squall — it  has  no  time  to 
pick  up  anything  from  the  earth's  rotation — and  produces 
the  squall  q,  marked  by  a  long  arrow. 

Our  section,  then,  of  a  squall  is  that  of  a  vertical 
whirl,  the  whole  system  perhaps  not  one  mile  high 
by  one  and  a  half  mile  across,  while  the  length  of  the 
front  of  the  storm  may  be  two  hundred  miles ;  and  our 
picture  of  the  whole  must  be  a  long,  nearly  straight 
horizontal  axis,  moving  broadside  on,  round  which  the 


254  WEATHER. 

wind  whirls  vertically  in  a  direction  opposite  to  that  of 
the  watch-hands.  It  is  always  an  episode,  as  it  were,  in 
the  history  of  some  general  form  of  atmospheric  circula- 
tion. We  can,  perhaps,  see  how  a  broad-fronted  west 
current  can  meet  a  southerly  stream  of  air  in  the  trough 
of  a  V  or  cyclone ;  but  we  are  unable  at  present  to  form 
any  conception  of  a  straight-fronted  current  advancing 
across  the  curved  isobars  of  an  anticyclone,  as  in  Fig.  52 
for  French  thunderstorms. 

Line-squalls  and  thunderstorms  of  a  different  type 
are  very  common  in  the  tropics.  The  author  has  observed 
a  very  striking  instance  at  the  junction  of  the  south-east 
trade  and  north-west  monsoon  in  the  Indian  Ocean. 
There  was  no  doldrum,  but  the  two  currents  met  along  a 
line  whose  position  was  marked  by  a  long  dark,  black 
cloud,  with  heavy  rain  and  squall. 

In  like  manner,  the  daily  thunderstorm  which  occurs 
in  so  many  countries  at  the  time  when  the  sea-breeze 
comes  in,  charging,  as  it  were,  the  prevailing  wind  over 
the  land,  is  due  to  a  long  vertical  whirl  where  the  two 
currents  meet,  and  the  whole  length  can  sometimes  be 
watched  gradually  advancing  inland  from  the  coast.  The 
so-called  north-westers  at  Calcutta,  during  the  hot  season, 
belong  to  this  last  type. 

THUNDERSTORMS  WITH  SECONDARIES. 

We  must  now  just  mention  a  class  of  thunderstorms 
which  are  more  complicated  than  a  simple  squall,  and 
yet  differ  in  many  ways  from  line-thunderstorms.  They 
are  associated  with  secondary  cyclones,  and  are  much 


SQUALLS,  THUNDERSTORMS,   NON-ISOBARIC  RAINS.      255 


commoner  in  England  than  line-thunderstorms,  but 
none  have  been  tracked  over  a  sufficiently  long  area  to 
allow  us  to  say  anything  about  their  shape  or  motion. 
All  we  know  is,  that  as  surely  as  we  see  a  secondary  on 
the  charts  in  summer,  so  certainly  will  thunderstorms 
occur  during  the  day,  though  we  cannot  say  in  what 
portion  of  the  small  depression. 

The  general  features  of  this  kind  of  disturbance  will 
be  best  understood  by  refer- 
rence  to  Fig.  55,  where  we 
give  a  typical  example  of 
that  distribution  of  pressure 
which  is  associated  with  a 
summer  thunderstorm  in 
Great  Britain,  The  isobars, 
wind-arrows,  and  weather- 
symbols  give  the  synoptic 
conditions  of  North-western 
Europe  at  8  a.m.  July  3, 
1883.  The  broad  features 
of  pressure-distribution  are 
found  in  an  anticyclone  over 
Scandinavia,  in  the  fragment 
of  a  large  depression  to  the 
west  of  Ireland,  and  in  a 
complicated  mass  of  second- 
aries over  Great  Britain  and  the  north  of  France.  The 
general  direction  of  the  wind  is  from  the  south,  but  both 
the  bends  in  the  isobars,  which  mark  out  the  position  of 
the  secondaries  over  Central  England  and  the  north 
of  France,  are  associated  with  a  considerable  deflection  of 


55. — Conditions   of  thunder- 
storms. 


256  WEATHER. 

the  wind.  In  both  instances  we  see  a  partial  circulation 
of  the  wind  round  a  central  spot  of  calm ;  the  latter 
stations  are  marked  by  the  symbol  of  a  circle  with  a  dot 
in  the  centre.  At  the  moment  when  the  chart  was  con- 
structed, three  thunderstorms  were  in  progress :  two  on 
the  east  coast  of  England — at  Shields  and  Spurn  Head — 
with  one  in  France,  at  L'Orient.  All  these  stations  are 
marked  with  the  symbol  t. 

The  special  features  of  this  class  of  thunderstorm  are 
the  calm  sultry  weather  with  which  they  are  associated, 
so  different  from  the  squall  of  a  line-thunderstorm,  and 
the  limited  rotation  of  the  surface-wind  during  the  pro- 
gress of  the  storm.  Another  very  remarkable  feature  is 
that  this  surface  circling  of  the  wind  extends  only  a  very 
short  distance  upwards,  and  whenever  a  glimpse  can  be 
caught  of  the  drift  of  the  upper  clouds,  they  are  found  to 
move  in  the  same  direction  throughout  the  whole  period 
of  the  disturbance. 

This  is  the  familiar  class  of  thunderstorm  which  we 
associate  with  sultry  weather,  and  with  the  thunder  coming 
against  the  wind.  As  the  secondary  approaches  any 
station,  the  wind  draws  more  or  less  in  towards  the  centre, 
and  recovers  its  former  direction  after  the  depression  has 
passed.  In  a  case  like  that  figured  in  the  diagram,  the 
motion  of  the  storm  as  a  whole  would  be  towards  the 
north-east,  and  the  rotation  of  the  wind  at  any  place 
would  depend  on  which  side  of  the  centre  the  station  lay. 
.  Squalls,  troughs,  and  secondaries  do  not  by  any  means 
exhaust  all  the  conditions  under  which  thunder  and 
lightning  may  be  developed,  even  within  the  limits  of 
Europe,  but  any  attempt  to  examine  into  these  other 


SQUALLS,  THUNDERSTORMS,   NON-ISOBARIC   RAINS.      257 

causes  would  be  beyond  the  scope  of  this  work.  We 
have,  therefore,  only  indicated  the  three  principal  sources 
of  electrical  discharge,  which,  however,  will  account  for 
more  than  eighty  per  cent,  of  European  thunderstorms. 
The  great  thing  is  to  realize  that  there  are  several  dif- 
ferent kinds  of  thunderstorms,  each  of  which  has  certain 
distinctive  features. 


GENERAL  KEMARKS. 

We  may,  perhaps,  conclude  this  short  notice  of 
thunderstorms  with  a  few  general  remarks  on  the  subject. 
One  of  the  first  things  which  must  strike  everybody  is, 
that  even  in  the  temperate  zone  some  countries  are  far 
more  ravaged  by  thunderstorms  than  others.  For  instance, 
France  suffers  more  than  any  other  part  of  Europe,  and 
England  the  least.  We  may  probably  find  at  least  two 
causes  which  modify  the  development  of  thunderstorms. 
In  the  first  place,  the  geographical  position  of  the  country 
relative  to  the  great  seasonal  areas  of  high  and  low 
pressure.  From  this  point  of  view  we  can  readily  see 
that  France  is  far  more  exposed  to  the  influence  of  small 
secondaries,  which  come  in  from  the  Atlantic,  and  which 
die  out  before  they  reach  Central  Europe,  than  any  other 
portion  of  that  continent.  If  we  look  at  the  large  charts 
which  we  give  in  our  chapter  on  Weather  Types,  we  can 
easily  understand  that  whenever  a  cyclone  leaves  the 
Atlantic  anticyclone  to  go  towards  the  north-east,  there 
must  be  left  a  somewhat  irregular  col  of  pressure  over 
France,  and  this  we  know  is  most  conducive  to  the 
formation  of  thunderstorms. 


258  WEATHEE. 

The  second  cause  which  may  modify  the  formation  of 
storms  is  the  amount  of  vapour  in  the  air.  We  know  by 
experiment  that  the  discharge  of  frictional  electricity  is 
very  much  modified  by  the  hygrometric  condition  of  the 
atmosphere.  In  England,  if  we  turn  a  machine  on  a 
damp  day,  the  electricity  will  escape  as  fast  as  it  is  made, 
and  no  discharge  can  be  obtained ;  on  a  dry  day,  on  the 
contrary,  sparks  can  easily  be  procured.  From  this 
analogy  we  can  easily  conceive  that  the  same  atmospheric 
disturbance  which  causes  a  violent  thunderstorm  in  the 
dry  climate  of  France,  would  either  discharge  its  elec- 
tricity noiselessly,  or  at  all  events  occasion  but  a  very 
feeble  storm,  in  the  damper  atmosphere  of  Great  Britain. 

No  doubt,  insulated  damp  air  is  just  as  good  a  non- 
conductor as  dry  air ;  and  the  silent  discharge  of  a 
frictional  machine  in  damp  weather  is  due  to  the  con- 
densation of  a  thin  layer  of  water  on  the  surface  of  the 
insulating  supports.  But  in  the  atmosphere  we  cannot 
conceive  absolute  insulation.  The  air  is  full  of  ice  or 
water-dust,  and  we  know  that  somehow  or  other  electrical 
discharge  is  easily  propagated  from  one  cloud  to  another. 

We  cannot  say  absolutely,  in  our  present  state  of 
knowledge,  how  far  electricity  is  only  a  secondary  pro- 
duct of  atmospheric  disturbance,  or  whether  it  ever  plays 
a  primary  part  in  making  a  storm.  As  far  as  we  can  see, 
however,  the  evidence  is  entirely  in  favour  of  the  idea 
that  electricity  is  only  a  secondary  phenomenon.  We 
cannot  suppose  that  an  abnormal  amount  of  electricity 
can  ever  be  developed  without  some  definite  cause,  and 
it  is  also  an  obvious  fact,  that  in  thundery  weather  there 
are  often  a  great  many  showers  without  thunder,  which 


SQUALLS,  THUNDERSTORMS,   NON-ISOBARIC   RAINS.      259 

only  differ  from,  those  with  thunder  and  lightning  in 
violence  or  intensity. 

On  the  other  hand,  it  is  almost  certain  that  the  dis- 
charge of  electricity  is  in  some  manner  associated  with 
the  formation  of  hail  and  very  large  rain-drops ;  but  at 
present  no  complete  explanation  can  be  given  of  the 
relation  between  these  two  phenomena. 

NON-ISOBARIC  BAINS. 

We  have  now  to  deal  with  the  most  unsatisfactory 
branch  of  modern  meteorology — the  nature  of  those  falls 
of  rain  that  are  not  associated  with  any  definite  shape 
of  isobars,  and  which  are  therefore  called  "  non-isobaric  " 
rains.  We  have  already  described  the  very  remarkable 
case  of  line-thunderstorms  whose  position  cannot  be 
detected  by  any  inspection  of  isobars ;  and  numerous 
other  cases  of  a  less  striking  nature  are  of  constant 
occurrence.  The  importance  of  this  classification  of  rains 
in  any  comprehensive  treatise  on  meteorology  may  be 
judged  from  the  fact  that  in  Great  Britain,  though  the 
bulk  of  winter  rain  is  cyclonic,  a  great  deal  of  summer 
rainfall  is  non-isobaric;  in  Continental  Europe  a  still 
larger  proportion  is  of  the  latter  character ;  so  are  most 
tropical  rains,  except  the  downpour  of  hurricanes ;  while 
the  whole  of  the  heavy  rain  on  the  equator,  and  all  that 
falls  in  the  doldrums,  is  also  absolutely  non-isobaric, 

THE  SOUTH-WEST  MONSOON. 

But  by  far  the  most  striking  non-isobaric  rain  in  the 
world  is  the  burst  of  the  south-west  monsoon  in  the 


260  WEATHER. 

Indian  Ocean.  Let  us  consider  the  course  of  the  seasons 
in  Ceylon,  and  correlate  them  with  changes  of  pressure 
over  India.  In  the  month  of  February  we  find  a  very 
shallow  stationary  depression — not  a  cyclone — over  Lower 
Bengal,  a  belt  of  high  pressure  stretching  across  the 
Bay  of  Bengal  from  Madras  to  Rangoon,  and  a  general 
diminution  of  pressure  from  that  belt  to  the  equator. 
From  this  we  might  reasonably  expect  what  we  find — 
light  south-west  wind  over  Lower  Bengal,  variable  breezes 
over  Madras,  and  a  light  north-east  monsoon  across 
Ceylon  ;  but  it  is  not  so  obvious  why  the  south-west  wind 
should  be  so  fine  and  dry  as  it  is.  The  low  pressure  over 
Bengal  gets  gradually  more  pronounced,  and  spreads  with 
its  accompanying  south-west  wind  slowly  southwards,  till 
Ceylon  is  embraced  within  its  sphere.  These  conditions 
are  most  pronounced  towards  the  end  of  May,  and  we  get 
the  dry,  nearly  cloudless,  hot  season  of  India  and  Ceylon, 
with  a  light  south-west  wind.  Then  the  sky  begins  to 
cloud  over,  and  suddenly  rain  bursts  in  a  series  of  terrific 
thunderstorms,  and  the  bad  wet  weather  continues  for 
two  or  three  months.  The  rain  begins  in  Ceylon,  and 
then  works  slowly  up  the  west  coasts  of  India  and  Burinah 
— omitting  Madras — till  Calcutta  and  Lower  Bengal  are 
reached,  three  or  four  weeks  later  than  Colombo.  Then 
we  are  met  by  the  strange  fact  that  this,  the  most  striking 
weather-change  in  the  whole  year,  is  associated  by  no 
change  in  the  shape  of  the  isobars.  We  give  in  our 
chapter  on  Weather  Types  two  diagrams  (Figs.  82  and 
83)  of  Indian  isobars  just  after  the  monsoon  has  burst 
over  Bombay  and  Calcutta.  The  only  difference  in  the 
isobars  then  and  a  fortnight  previously,  when  the  hot  dry 


SQUALLS,   THUNDERSTORMS,   NON-ISOBARIC   RAINS.     261 

season  prevailed,  is  that  the  level  of  pressure  is  a  little 
lower,  that  the  position  of  lowest  barometer  has  moved 
a  little  higher  up  the  Ganges,  and  that  the  distortion  of  the 
isobars  by  secondaries  is  more  pronounced.  When  the 
monsoon  is  fairly  established,  we  can,  no  doubt,  see  certain 
slight  fluctuations  in  the  shape  and  intensity  of  the 
isobars  which  accompany  what  is  called  "  a  break  in 
the  rains,"  and  sometimes  exceptionally  heavy  rain  falls 
during  the  passage  of  a  small  cyclone  from  the  Bay  of 
Bengal  up  country ;  but  we  cannot  find  any  change  in 
the  isobars  to  account  for  the  sudden  change  of  weather 
which  is  called  in  common  parlance,  "  the  burst  of  the 
monsoon."  The  quality  of  the  rain,  if  nothing  else,  dis- 
tinguishes the  monsoon  from  cyclonic  precipitation.  The 
rain  in  front  of  a  Bengal  cyclone  seems  to  grow  out  of 
the  air,  while  that  of  the  monsoon  falls  in  thunderstorms 
and  from  heavy  cumuloform  clouds.  The  only  rational 
suggestion  which  has  been  made  to  account  for  this  burst 
of  rain,  would  look  to  a  sudden  inrush  of  damp  air  from 
the  region  of  the  doldrums  as  the  source  of  the  change  in 
weather,  but  not  of  the  direction  of  the  wind,  or  of  the 
shape  of  the  isobars ;  for  the  burst  is  apparently  almost 
coincident  with  the  disappearance  of  the  belt  of  high 
pressure  to  the  south  of  the  Bay  of  Bengal. 

No  satisfactory  clue  has,  however,  yet  been  discovered 
either  to  the  cause,  or  still  less  to  the  quantity,  of  non- 
isobaric  rainfalls.  They  are  the  bugbear  of  every 
European  forecaster,  though  in  Japan,  curiously  enough, 
they  find  rain  easier  to  announce  than  the  direction  of 
the  wind.  Mr.  Finley,  of  the  United  States  Signal  Office, 
has  made  some  very  interesting  studies  on  local  rains 


262  WEATHEK. 

that  do  not  show  on  the  isobaric  charts.  He  takes  a  map 
of  the  United  States,  and  puts  in  all  the  wind-arrows 
without  any  isobars.  Very  often  he  finds  some  large 
areas  swept  by  a  generally  southerly  wind,  and  others  by 
a  generally  northerly  wind,  and  he  draws  lines  to  mark 
out  the  tracts  of  country  where  these  currents  meet,  and 
where  they  diverge.  Then  he  finds  that  there  are  always 
local  rains  over  the  first  areas,  and  rarely  any  over  the 
latter.  This  would  undoubtedly  point  to  local  vertical 
whirls  between  the  meeting  currents  as  the  source  of  rain. 
Whether  this  is  universally  the  case,  or  whether  the 
conditions  of  all  rains  could  be  analyzed  into  small  Y's 
or  secondaries  if  the  isobars  were  constructed  from  stations 
sufficiently  close  together,  we  cannot  at  present  say. 
The  important  thing  is  not  to  mix  up  all  kinds  of  rain 
together  when  we  want  to  discuss  general  meteorological 
problems. 


(    263    ) 


CHAPTER  IX. 

PAMPEKOS,  WHIRLWINDS,   AND   TORNADOES. 

WE  will  now  describe  two  remarkable  kinds  of  storms 
which  occur  in  La  Plata  and  in  the  United  States 
respectively. 

PAMPEROS. 

The  word  "  pampero  "  is,  unfortunately,  used  in  a  very 
vague  manner  in  the  Argentine  Republic  and  neigh- 
bouring states.  Every  south-west  wind  which  blows  from 
off  the  pampas  is  sometimes  called  a  pampero  ;  and  there 
is  a  still  further  confusion  caused  by  calling  certain  dry 
dust-storms  pamperos  sucios,  or  dry  pamperos.  The  true 
pampero  may  be  described  as  a  south-west  wind,  ushered 
in  by  a  sudden  short  squall,  usually  accompanied  by  rain 
and  thunder,  with  a  very  peculiar  form  of  cloud-wreath. 
We  will  describe  these  as  given  by  D.  Christison  in  the 
Proceedings  of  the  Scottish  Meteorological  Society,  No.  Ix. 
p.  330,  and  then  we  shall  have  no  difficulty  in  recognizing 
a  line-squall  as  the  source  of  the  pampero. 

The  barometer  always  falls  pretty  steadily  for  from 


2G4  WEATHEK. 

two  to  four  days  before  the  pampero,  and  always  rises 
for  some  days  after  the  squall.  There  are  not  enough 
barometric  observations  to  allow  of  any  generalizations 
as  to  the  precise  position  of  the  squall  relative  to  the 
trough  of  the  general  depression,  but  in  two  recorded 
cases  the  mercury  began  to  rise  some  hours  before  the 
storm  burst. 

Temperature  is  always  very  high  before  the  squall, 
and  then  the  sudden  change  of  wind  sends  the  thermo- 
meter rapidly  down,  sometimes  as  much  as  33°  in  six 
hours. 

Thunder  accompanies  about  three  out  of  four  pamperos ; 
but  more  or  less  rain  always  falls,  except  in  the  rarest 
cases. 

The  wind  before  this  class  of  pampero  almost  in- 
variably blows  moderately  or  gently  for  some  days  from 
easterly  points,  and  then  with  a  sudden  burst  the  south- 
west wind  comes  down  with  its  full  strength,  and,  after 
blowing  thus  from  ten  to  thirty  minutes,  either  ceases 
entirely  or  continues  with  diminished  force  for  a  certain 
number  of  hours.  In  all  cases  but  one  the  upper  wind- 
currents  have  been  seen  to  come  from  the  north-west  both 
before,  during,  and  after  the  pampero. 

The  general  appearance  of  a  pampero  will  be  best 
understood  by  a  description  of  an  actual  squall.  "  In  the 
early  morning  of  a  day  in  November,  the  wind  blew 
rather  strongly  from  the  north-east.  The  sky  was  cloudy, 
but  not  overcast,  save  in  the  south-west  horizon.  The 
clouds  were  moving  very  slowly  from  the  west,  or  a  little 
south  of  it,  throwing  out  long  streamers  eastwards. 
About  8  a.m.  the  threatening  masses  in  the  south-west 


PAMPEROS,  WHIRLWINDS,  AND  TORNADOES.  265 

had  advanced  near  enough  to  show  that  at  their  head 
marched  two  dense  and  perfectly  regular  battalions  of 
cloud,  one  behind  the  other,  in  close  contact,  yet  not 
intermingling,  and  completely  distinguished  by  their 
striking  difference  of  colour,  the  first  being  of  a  uniform 
leaden  grey,  while  the  second  was  as  black  as  the  smoke 
of  a  steamer.  On  arriving  overhead,  it  was  seen  that  the 
front,  although  slightly  sinuous,  was  perfectly  straight  in 
its  general  direction,  and  that  the  bands  were  of  uniform 
breadth.  As  they  rushed  at  a  great  speed  under  the 
other  clouds  without  uniting  with  them,  preserving  their 
own  formation  unbroken,  their  force  seemed  irresistible, 
as  if  they  were  formed  of  some  solid  material  rather  than 
vapour.  The  length  of  these  wonderful  clouds  could  not 
be  conjectured,  as  they  disappeared  beneath  the  horizon 
at  both  ends,  but  probably  at  least  fifty  miles  of  them 
must  have  been  visible,  as  the  '  Cerro '  commands  a 
view  of  twenty  miles  of  country.  Their  breadth  was  not 
great,  as  they  only  took  a  few  minutes  to  pass  overhead, 
and  appeared  to  diminish  from  the  effects  of  perspective 
to  mere  lines  on  the  horizon.  At  the  instant  when  the 
first  band  arrived,  the  wind — which  was  still  blowing,  and 
something  more  than  gently,  from  the  north-east — went 
round  by  north  to  south-west ;  at  the  same  time  a  strong, 
cold  blast  fell  from  the  leaden  cloud,  and  continued  to 
blow  till  both  bands  had  passed.  From  neither  of  them, 
however,  came  lightning  or  rain,  but,  filling  up  the  sky 
in  rear  of  the  regular  army,  followed  a  confused  rabble  of 
clouds,  with  a  constant  rumbling  of  thunder,  and  from 
which  evidently  rain  was  falling.  It  was  not,  however, 
till  fifteen  minutes  after  the  passage  of  the  two  regular 


266 


WEATHER. 


bands  that  rain  fell  where  the  observations  were  taken. 
The  storm,  passing  on,  obscured  the  whole  sky,  wind,  rain, 
and  thunder  continuing  for  some  hours,  but  only  to  a 
moderate  degree."  The  diagram  (Fig.  56),  taken  from  a 
sketch  made  at  the  time,  represents  the  northerly  half  of 
the  storm-clouds  while  still  at  some  distance  from  the 
spectator,  and  advancing  from  a  westerly  direction. 

From  all  this  it  is  manifest  that  the  changes  of  wind, 
the  rapid  alternations  of  temperature,  and  the  typical 
cloud-wreaths  are  identical  in  character  with  the  class  of 


FIG.  56. — Cloud-wreath  iu  pampero. 


disturbance  we  have  described  in  the  previous  chapter 
under  the  heading  of  Line  Squalls  and  Line  Thunder- 
storms. We  must  remember,  however,  that  being  south 
of  the  equator,  north-east,  south-west,  and  north-west 
winds  are  equivalent  to  those  from  south-east,  north-west,, 
and  south-west  in  the  northern  hemisphere. 


PAMPEROS,  WHIRLWINDS,  AND  TORNADOES.         267 

WHIRLWINDS. 

A  whirlwind  may  be  described  as  a  mass  of  air  whose 
height  is  enormously  greater  than  its  width,  rotating 
rapidly  round  a  more  or  less  vertical  axis.  A  moderate 
whirlwind  may  be  two  hundred  feet  high,  and  not  above 
ten  feet  in  diameter.  The  dimensions,  however,  are  very 
variable,  for  a  whirlwind  may  vary  in  intensity  from  a 
harmless  eddy  in  a  dusty  road  to  the  destructive  tornado 
of  the  United  States.  As  the  latter  are  the  most  terrific 
manifestation  of  weather  in  the  whole  range  of  meteoro- 
logy, we  shall  devote  a  few  pages  to  their  consideration. 

TORNADOES. 

A  tornado  is  simply  a  whirlwind  of  exceptional 
violence  ;  if  it  were  to  encounter  a  lake  or  the  sea,  it 
would  be  called  a  waterspout.  Its  most  characteristic 
feature  is  a  funnel,  or  spout,  which  is  the  visible  manifes- 
tation of  a  cylinder  of  air  that  is  revolving  rapidly  round 
a  nearly  vertical  axis.  This  spout  is  propagated  through- 
out the  northern  temperate  zone  in  a  north-easterly 
direction  at  a  rate  of  about  thirty  miles  an  hour,  and 
tears  everything  to  pieces  along  its  narrow  path. 

The  diameter  of  the  actual  spout  often  does  not 
exceed  a  few  yards,  and  the  total  area  of  destructive 
wind  is  rarely  more  than  three  or  four  hundred  yards 
across.  The  height  of  the  spout  is  that  of  the  lowest 
layer  of  clouds,  which  are  then  never  high ;  and,  as  in 
thunderstorms,  the  upper  currents  are  unaffected  by  the 
violent  commotion  below. 


268  WEATHEK. 

The  spout  as  a  whole  has  four  distinct  motions : 

1.  A   motion   of    translation   generally   towards    the 
north-east  at  a  variable  rate,  but  which  may  be  taken  to 
average  thirty  miles  an  hour. 

2.  A  complex  gyration.     The   horizontal  portion  of 
this  rotation  is  always  in  a  direction  opposite  to  that  of 
the  hands  of  a  watch — that  is  to  say,  in  the  same  manner 
as  an  ordinary  cyclone.     But  in  addition  to  this  there  is 
a  violent  upward  current  in  the  centre  of  the  cylinder  of 
vapour  or  dust  which  constitutes  the  spout,  and  sometimes 
small  clouds  seem  to  dart  down  the  outer  sides  of  the 
funnel  whenever  these  float  in  close  proximity.     There 
are,  however,  no  authentic  instances  of  any  object  being 
thrown  to  the  ground  by  the  individual  effort  of  a  down- 
ward current.     The  slight   downward   motion   of  a  few 
small  clouds  is  probably  only  a  slight  eddying  of  a  violent 
nprush. 

3.  A  swaying  motion  to  and  fro  like  a  dangling  whip, 
or  an  elephant's  trunk,  though  the  general  direction  of 
the  spout  is  always  vertical. 

4.  A  rising  and  falling  motion,  that  is  to  say,  that 
sometimes  the  end  of  the  funnel  rises  from  the  surface  of 
the  ground  and  then  descends  again,  and  so  on.     Owing 
to  this  rise  and  fall,  the  general  appearance  of  the  tornado 
changes  a  good  deal.     When  the  bottom  of  the  spout  is 
some  distance  above  the  ground,  the  whole  is  somewhat 
pointed,  and  does  comparatively  little  harm  as  it  passes 
over  any  place.     As  the  spout   descends,  a  commotion 
commences  on  the  surface  of  the  ground.     This  latter 
gradually  rises  so  as  to  meet  the  descending  part  of  the 
spout,  ami  then  the  whole  takes  the  shape  of  an  hour- 


PAMPEROS,   WHIRLWINDS,  AND  TORNADOES.  269 

glass,  as  in  Fig.  57.  This  is  the  most  dangerous  and 
destructive  form,  because  the  ground  gets  the  whole  force 
of  the  tornado. 

The  general  appearance  of  the  cloud  over  a  tornado 
or  whirlwind  is  always  described  as  peculiarly  smoky,  or 


FIG.  57. — Tornado-cloud. 

like  the  fumes  of  a  burning  haystack.  The  tornado  is 
also  never  an  isolated  phenomenon ;  it  is  always  associated 
with  rain  and  electrical  disturbance. 

The  destructive  effects  of  the  tornado  are  very  curious, 
from  the  sharp  and  narrow  belt  to  which  the  injury  is 
confined.  It  appears  that  in  the  passage  of  some 
tornadoes  wind-pressures  of  various  amounts,  from  eighteen- 


270  WEATHER. 

to  a  hundred  and  twelve  pounds  per  square  foot,  have 
been  demonstrated  by  destruction  of  bridges,  brick  build- 
ings, etc.  The  upward  pressures  are  sometimes  as 
great  as  the  horizontal,  and  even  greater.  Downward 
pressures  or  movements  of  wind  have  not  been  clearly 
proved.  Upward  velocities  of  135  miles  per  hour  seem 
not  to  be  unusual,  and  horizontal  velocities  of  eighty 
miles  have  been  recorded  with  the  anemometer.  The 
destructive  wind- velocities  are  confined  to  very  small 
areas.  A  destruction  of  fences,  trees,  etc.,  is  often  visible 
over  a  path  many  miles  long  and  a  few  hundred  yards 
•wide,  but  the  path  of  greatest  violence  is  very  much 
narrower.  The  excessive  cases  above  referred  to  are 
observed  only  in  small  isolated  spots,  less  than  a  hundred 
feet  square,  unequally  distributed  along  the  middle  of 
the  track.  Thus,  in  very  large  buildings,  only  a  small 
part  is  subject  to  destructive  winds.  In  different  parts 
of  this  area  of  maximum  severity,  the  winds  are  simul- 
taneously blowing  in  different,  perhaps  opposite,  direc- 
tions, the  resultant  tending  not  to  overturn  or  carry  off 
or  crush  in,  but  rather  to  twist  round  a  vertical  axis. 
Buildings  are  generally  lifted  and  turned  round  before 
being  torn  to  pieces.  As  the  chances  are  very  small  that 
a  building  will  be  exposed  to  the  violent  twisting  action, 
it  is  evidently  the  average  velocity  of  rectilinear  winds 
within  the  path  of  moderate  destruction  that  it  is  most 
necessary  to  provide  against  in  ordinary  structures.  These 
winds  may  attain  a  velocity  of  eighty  miles  an  hour  over 
an  area  of  a  thousand  feet  broad,  and  generally  blow  from 
the  south-west ;  the  next  in  frequency  blow  from  the 
north-west.  The  time  during  which  an  object  is  exposed 


PAMPEROS,   WHIRLWINDS,   AND  TORNADOES.  271 

to  the  more  destructive  winds  varies  from  six  to  sixty 
seconds.  An  exposed  building  experiences  but  one  stroke, 
like  the  blow  of  a  hammer,  and  the  destruction  is  done. 
Hence,  in  a  suspension-bridge,  chimney,  or  other  structure 
liable  to  be  set  into  destructive  rhythmic  vibrations,  the 
maximum  winds  do  not  produce  such  vibrations.  The 
duration  of  the  heavy  south-west  or  north-west  winds 
over  the  area  of  moderate  destruction  is  rarely  over  two 
minutes.  The  motion  of  translation  of  the  central  spout 
of  a  tornado,  in  which  there  is  a  strong  vertical  current, 
is,  on  an  average,  at  the  rate  of  thirty  miles  an  hour. 

Tornadoes  mostly  occur  on  sultry  days  and  either  in 
the  south-east  or  right  front  of  cyclones,  or  in  front  of 
the  trough  of  V-depressions. 

The  relative  frequency  of  tornadoes  is,  in  order  of  de- 
creasing frequency,  June,  July,  April,  May,  .  .  .  January. 
In  the  geographical  distribution  of  247  tornadoes  from 
1794  to  1878,  the  largest  figures  are  obtained  from  New 
York  (24),  Indiana  (20),  Illinois  (20),  Ohio,  and  Georgia 
(16  each),  etc. ;  but  the  records  are  fragmentary,  and  now 
Kansas  is  the  most  tornado-stricken  state.  The  largest 
number  of  tornadoes  apparently  occur  between  5  p.m.  and 
6  p.m. ;  the  next  between  4  p.m.  and  5  p.m. 

The  following  example  will  illustrate  all  the  principal 
features  of  tornadoes.  It  is  taken  from  one  of  the  reports 
of  the  United  States  Signal  Office,  and  the  author  is 
indebted  to  the  chief  signal-officer  for  supplying  him 
with  the  materials.  The  most  important  part  of  the 
work  has  been  done  by  Mr.  Finley  of  that  office,  who 
for  many  years  has  levoted  his  special  attention  to  the 
subject. 


272 


WEATHER. 


In  Fig.  58  we  give  a  synoptic  chart  of  the  meteoro- 
logical conditions  of  a  portion  of  the  United  States  on 
May  5,  1879,  at  4.35  p.m.,  Washington  time.  The  course 
of  the  great  rivers  Mississippi,  Missouri,  and  Ohio,  together 
with  the  outline  of  Lake  Michigan,  are  clearly  marked 


Local. 


3-55-P-™- 


FIG.  58. — Conditions  and  paths  of  tornadoes. 

by  thin  lines ;  and  the  position  of  the  cities  of  Chicago, 
St.  Louis,  Cincinnati,  Cairo,  and  Montgomery  will  be 
readily  recognized  by  their  initial  letters ;  so  that  there 
is  no  need  to  indicate  the  boundaries  of  any  state.  The 
isobars  are  shown  by  bold  black  lines,  and  the  arrows 


PAMPEROS,  WHIRLWINDS,  AND  TOENADOES.  273 

fly  with  the  wind  at  the  different  stations.  No  less  than 
eleven  tornadoes  ravaged  the  Western  States  during  that 
day;  the  course  of  each  of  these  is  shown  on  the  map 
by  a  line  of  small  circles,  joined  together,  and  those 
numbered  1,  2,  3,  and  4  respectively  were  apparently 
actually  in  progress  at  the  moment  to  which  the  chart 
refers.  Local  time  is  indicated  by  figures  on  the  top  of 
the  diagram. 

The  broad  features  of  pressure-distribution  are  suffi- 
ciently simple.  One  area  of  high  pressure  lies  over 
Manitoba ;  another  over  the  southern  states ;  a  col  lies 
between  them.  On  the  previous  day  this  col  had  been 
filled  by  a  V,  which  by  this  morning  had  partially 
developed  into  the  secondary  cyclone  which  now  lies  over 
the  Western  States.  A  portion  of  the  original  V  is  still 
seen  just  to  the  west  of  Lake  Michigan.  The  trough  of 
the  V  is  undoubtedly  connected  with  the  trough  of  the 
secondary,  but  the  latter  appears  to  be  made  up  of  several 
subsidiary  depressions ;  the  general  direction  of  the  wind 
is  typical  of  such  isobars.  In  front  of  the  V  and  secondary 
the  general  sweep  of  the  wind  is  from  the  south — a  little 
more  south-east  in  some  places,  and  a  little  more  south- 
west in  others. 

In  rear  of  the  trough,  the  general  direction  of  the 
wind  is  northerly — a  little  more  north-east  in  some  places, 
and  a  little  more  north-west  in  others.  The  formation 
of  tornadoes  appears  to  have  been  associated  everywhere 
with  the  secondary,  but  at  the  moment  for  which  the 
chart  is  constructed,  the  tornado  marked  4  seems  to  have 
been  produced  by  a  different  disturbance  from  that  which 
caused  numbers  1,  2,  and  3.  If  we  had  more  barometric 


274  WEATHER. 

observations,  we  should  most  probably  find  that  the 
projection  of  the  isobar  of  29 •?  ins.  which  surrounds 
number  4  tornado,  was  really  a  separate  subsidiary 
depression.  It  should  also  be  noticed  that  all  the  eleven 
tornadoes  moved  in  the  same  general  direction,  which  was 
practically  identical  with  that  of  the  whole  system  of 
depressions. 

The  general  character  of  all  tornadoes  is  so  similar 
that  the  description  of  one  will  do  for  all.  We  shall 
therefore  give  some  of  the  description  furnished  by  an 
eye-witness  to  the  United  States  Signal  Office,  of  the 
tornado  marked  3  in  the  chart  (Fig.  58),  which  is  de- 
scribed in  the  reports  as  the  "  Delphos  "  tornado. 

"  On  Friday  morning,  May  30,  1879,  the  weather  was 
very  pleasant,  but  warm,  with  the  wind  from  the  south- 
east, from  which  direction  it  had  blown  for  several  days. 
The  ground  was  very  dry,  and  no  rain  had  fallen  for  a 
number  of  weeks.  About  2  p.m.  threatening  clouds 
appeared  very  suddenly  in  the  west  (against  the  wind), 
attended  in  a  few  minutes  by  light  rain,  the  wind  still  in 
the  south-east.  It  stopped  in  about  five  minutes,  and 
then  commenced  again,  wind  still  the  same,  accompanied 
by  hail,  which  was  thick  and  small  at  first,  but  rapidly 
grew  less  in  quantity  and  larger  in  size,  some  stones 
measuring  three  and  a  half  inches  in  diameter,  and  one 
was  found  weighing  one-fourth  of  a  pound.  This  last 
precipitation  continued  for  about  thirty  minutes,  after 
which  a  cloud  in  the  shape  of  a  water-spout  was  seen 
forming  in  the  south-west,  and  moving  rapidly  forward  to 
the  north-east.  The  cloud  from  which  the  funnel  de- 
pended, seen  at  a  distance  of  eight  miles,  appeared  to  be 


PAMPEROS,  WHIRLWINDS,  AND  TORNADOES.  275 

in  terrible  commotion ;  in  fact,  while  the  hail  was  falling, 
a  sort  of  tumbling  in  the  clouds  was  noticed  as  they 
came  up  from  the  north-west  and  south-west,  and  about 
where  they  appeared  to  meet  was  the  point  from  which 
the  funnel  was  seen  to  descend.  There  was  but  one 
funnel  at  first,  which  was  soon  accompanied  by  several 
smaller  ones,  dangling  down  from  the  overhanging  clouds 
like  whip-lashes,  and  for  some  minutes  they  were  appear- 
ing and  disappearing  like  fairies  at  a  play.  Finally  one 
of  them  seemed  to  expand  and  extend  downwards  more 
steadily  than  the  others,  resulting  at  length  in  what 
appeared  to  be  their  complete  absorption.  This  funnel- 
shaped  cloud  now  moved  onward,  growing  in  power  and 
size,  whirling  rapidly  from  right  to  left,  rising  and 
descending,  and  swaying  from  side  to  side.  When  within 
a  distance  of  three  or  four  miles,  its  terrible  roar  could  be 
heard,  striking  terror  into  the  hearts  of  the  bravest."  The 
eye- witness  judged  that  the  funnel  itself  would  reach  a 
height  of  about  five  hundred  feet  from  the  ground.  As 
the  storm  crossed  a  river,  a  cone-shaped  mass  came  up 
from  the  earth  to  meet  it,  carrying  mud,  debris,  and  a 
large  volume  of  water  (Fig.  57).  The  cloud  then  passed 
the  observer's  house  very  near  to  4  p.m.  The  progressive 
velocity  at  the  time  was  considered  to  be  about  thirty 
miles  per  hour,  although  at  Delphos,  three  and  a  half 
miles  distant,  it  had  slackened  down  to  near  twenty  miles. 
A  few  minutes  previous  to  and  during  the  passage  of  the 
funnel,  the  air  was  very  oppressive ;  but  ten  minutes 
after,  the  wind  was  so  cold  from  the  north-west  that  it 
became  necessary  to  wear  an  overcoat  when  outside. 
As  the  tornado  struck  the  house,  another  member  of 


276  WEATHER, 

the  family  says,  "  We  think  it  is  coming  near  us.  We 
can  now  see  its  fury.  Shall  we  leave  the  house  ?  No ; 
for  we  are  not  certain  on  which  side  it  will  pass.  We  are 
apparently  as  safe  here  as  elsewhere.  The  windows  are 
nailed  fast.  Three  of  us  lean  against  the  door  which  is 
nearest  the  storm;  the  rest  go  into  the  cellar.  It  is 
about  4  p.m.  A  moment  of  breathless  suspense,  and  the 
storm  strikes  us.  The  timbers  creak,  the  sides  of  the 
house  sway  in  and  out;  surely  they  cannot  outlast  it. 
We  hear  no  well-defined  roar  now,  for  on  the  outside 
boards  and  other  debris  are  fiercely  crashing.  All  is  dark 
within.  In  about  fifteen  minutes  the  storm  is  over.  We 
leave  the  house.  The  centre  of  the  storm  has  passed  to 
the  west  of  us,  and  we  can  see  its  dark  form  moving  away 
in  a  north-east  direction." 

The  actual  diameter  of  this  storm  appears  to  have  been 
only  forty-three  yards.  On  the  right  of  the  track,  destruc- 
tive winds  extended  to  a  further  distance  of  from  one  to 
two  miles,  sensible  deflected  winds  for  another  mile  and  a 
half,  beyond  which  only  the  usual  wind  of  the  day  was 
experienced.  On  the  left  or  northern  side  of  the  tornado 
path,  the  damage  did  not  extend  quite  so  far,  for  the 
width  of  the  belt  of  destructive  winds  was  not  more  than 
twenty-eight  yards  across,  and  that  of  sensibly  deflected 
winds  one  mile  and  a  quarter. 

As  a  specimen  of  the  damage  done,  a  large  two-horse 
sulky  plough,  weighing  about  seven  hundred  pounds,  was 
carried  a  distance  of  twenty  yards,  breaking  off  one  of 
the  iron  wheels  attached  to  an  iron  axle  one  and  three- 
quarter  inches  in  diameter.  A  woman  was  carried  to  the 
north-west  two  hundred  yards,  lodged  against  a  barbed- 


PAMPEROS,  WHIRLWINDS,  AND  TORNADOES.          277 

wire  fence,  and  instantly  killed.  Her  clothing  was  entirely 
stripped  from  her  body,  which  was  found  covered  with 
black  mud,  and  her  hair  matted  with  it.  A  cat  was 
found  half  a  mile  to  the  north-west  of  the  house,  in  which 
she  had  been  seen  just  before  the  storm,  with  every  bone 
broken.  Chickens  were  stripped  of  their  feathers,  and 
one  was  found  three  miles  to  the  north-west. 

A  few  miles  further  on,  another  eye-witness  says, 
"  The  dark,  inky,  funnel-shaped  cloud  rapidly  descended 
to  the  earth,  which  reaching,  it  destroyed  everything 
within  its  grasp.  Everything  was  taken  up  and  carried 
round  and  round  in  the  mighty  whirl  of  the  terrible 
monster.  The  surrounding  clouds  seemed  to  roll  and 
tumble  towards  the  vortex. 

"  The  funnel,  now  extending  from  the  earth  upwards 
to  a  great  height,  was  black  as  ink,  excepting  the  cloud 
near  the  top,  which  resembled  smoke  of  a  light  colour. 
Immediately  after  passing  the  town,  there  came  a  wave 
of  hot  air,  like  the  wind  blowing  from  a  burning  building. 
It  lasted  but  a  short  time.  Following  this  peculiar 
feature,  there  came  a  stiff  gale  from  the  north-west,  cold 
and  bleak,  so  much  so  that  during  the  night  frost 
occurred,  and  water  in  some  low  places  was  frozen." 

EELATION  OF  WHIRLWINDS  TO  CYCLONES. 

Before  concluding  this  chapter,  we  may  make  a  few 
remarks  on  a  very  interesting  question  which  here  pre- 
sents itself.  Commencing  with  a  whirlwind  only  two  or 
three  feet  across,  we  find  every  gradation  of  size  till 
we  come  to  the  destructive  tornado.  From  the  small 


278  WEATHER. 

secondary  which  deflects  the  wind  in  connection  with  a 
thunderstorm,  there  seems  to  be  every  gradation  of  size 
into  the  secondary  which  is  so  large  that  we  can  hardly 
say  whether  it  should  not  be  called  a  primary  cyclone. 

In  both  the  whirlwind  and  cyclone  series  we  have 
certain  common  features — a  horizontal  rotation,  and  more 
or  less  uptake  near  the  centre  of  the  gyration.  But  is 
there  any  intermediate  series  between  the  whirlwinds  and 
the  cyclones,  or  can  the  former  ever  develop  into  the 
latter  ?  We  believe  not,  though  the  opposite  opinion  has 
often  been  propounded. 

In  the  first  place,  we  are  unable  to  find  any  connecting 
link  between  the  two  types  of  rotation.  Under  certain 
conditions,  the  wind  seems  to  have  a  tendency  to  form 
little  eddies,  which  under  favourable  circumstances  may 
grow  into  complete  cylinders  of  rotating  dust.  In  our 
chapter  on  Prognostics  we  have  shown  that  in  England 
whirling  dust  is  a  well-known  precursor  of  showers  of 
rain,  but  not  of  the  true  cyclone-rain.  In  other  countries, 
such  as  the  Punjab  and  on  the  Isthmus  of  Suez,  regular 
whirlwinds  are  of  daily  occurrence  at  certain  seasons  of 
the  year ;  but  these  never  by  any  chance  grow  into  even 
the  smallest  secondary.  We  have  just  seen  that  the 
terrific  tornadoes  of  the  Western  States  of  the  American 
Union  are  merely  an  episode  in  the  conflict  of  opposing 
currents  near  the  trough  of  larger  depressions,  but  the 
whirlwinds  never  give  rise  to  any  larger  disturbance.  In 
every  one  of  the  eleven  tornadoes  which  occurred  on  the 
day  we  have  just  described  in  some  detail,  it  was  found 
that  rain  and  hail  invariably  preceded  the  tornado-cloud 
from  ten  to  thirty  minutes,  and  that  the  tornado  was 


PAMPEROS,  WHIRLWINDS,  AND  TORNADOES.  279 

only,  as  it  were,  a  local  accident  in  a  very  large  dis- 
turbance. 

In  our  chapter  on  Weather-types,  we  shall  give 
abundant  illustrations  of  the  manner  in  which  both 
primary  and  secondary  cyclones  are  formed  without  the 
presence  of  two  such  opposing  currents  as  are  found  in 
front  and  rear  of  the  trough  of  a  Y-depression,  and  we 
shall  see  how  the  two  kinds  of  cyclones  may  either 
develop  or  degrade  from  one  into  the  other. 

We  have  also  already  seen  the  very  small  circulation 
of  the  wind  which  accompanies  some  kinds  of  thunder- 
storms, but  in  no  case  do  we  find  any  transitional  link 
between  the  whirlwind  and  cyclone  types  of  rotatory 
motion. 

At  the  same  time,  we  may  note  that  the  inner  core 
and  very  deep  central  depression  of  a  tropical  hurricane 
approximates  more  nearly  to  the  tornado  type  than  the 
cyclones  of  temperate  regions ;  but  the  absence  of  tran- 
sitional forms  seems  conclusive  against  the  identity  of 
tornadoes  and  cyclones.  In  both  the  destructive  fury  is 
out  of  all  keeping  with  any  forces  that  we  are  acquainted 
with  ;  and  their  true  nature  remains  to  be  discovered  by 
future  research. 


. 
.  • 


280  WEATHER, 


CHAPTER  X. 

LOCAL  VARIATION  OF  WEATHER. 
NATURE  AND  PRINCIPLES. 

THE  object  of  this  chapter  is  to  explain  what  is  known  as 
the  local  variation  of  weather.  This  term  groups  con- 
veniently a  large  class  of  dependent  phenomena,  which 
owe  their  origin  to  the  influence  of  local  obstacles  or 
peculiarities  on  the  development  of  weather.  We  know 
that  in  the  same  country  some  places  are  much  colder  or 
wetter  than  others;  that  some  are  more  exposed  to 
destructive  gales;  and  that  others  are  more  frequently 
ravaged  by  disastrous  hailstorms.  We  will  now  en- 
deavour to  show  why  this  should  be  so,  and  how  the  pro- 
ducts of  this  variation  are  related  to  the  general  principles 
of  the  dependence  of  weather  on  the  distribution  of 
atmospheric  pressure  which  we  have  already  described  so 
fully. 

If  we  watch  the  actual  occurrence  of  any  local 
peculiarity  of  weather,  we  shall  soon  find  out  that  in  every 
instance  it  is  the  intensity,  and  not  the  general  character, 
which  is  altered.  For  instance,  two  places  a  few  miles 


LOCAL   VARIATION  OF  WEATHER.  281 

apart  may  differ  by  10°  of  temperature  on  a  frosty 
morning.  No  local  cause  has  formed  the  distribution 
of  pressure  which  gives  the  necessary  calm.  That  stillness 
has  been  developed  by  general  causes,  while  it  is  local 
peculiarities  of  exposure,  etc.,  which  have  enabled  radia- 
tion to  be  so  much  more  powerful  in  one  place  than 
another.  Similarly,  an  inch  of  rain  may  fall  in  one  place, 
and  only  a  few  drops  in  another  not  far  distant.  But  if  we 
think  of  the  day  on  which  this  occurred,  we  shall  remember 
that  it  was  cloudy  and  showery  naturally.  The  difference 
of  actual  rainfall  was  either  due  to  one  place  catching  a 
heavy  shower  which  did  not  affect  the  other,  or  else  some 
local  peculiarity  of  the  one,  which  increased  the  amount 
of  precipitation  that  would  otherwise  have  been  induced 
by  a  cyclone  of  any  given  intensity. 

These  instances  might  be  multiplied  indefinitely,  and 
it  is  from  the  observation  of  innumerable  cases  that  we 
are  enabled  to  lay  down  the  general  law  that  the  primary 
character  of  all  weather  is  given  by  the  general  distribu- 
tion of  surrounding  pressure ;  the  local  variation  modifies 
but  never  alters  this  general  character.  By  this  means 
we  are  able  to  steer  our  way  through  many  intricacies  of 
weather  which  would  otherwise  present  hopeless  diffi- 
culties, and  to  explain  many  phenomena  which  would 
otherwise  be  inexplicable.  Hence  we  see  the  appropriate- 
ness of  the  word  "  variation,"  as  applied  to  the  modification 
due  to  local  causes. 

The  cases  which  present  themselves  in  practice  are 
endless.  Every  country,  every  part  of  a  country,  has  a 
set  belonging  to  itself,  and  the  local  meteorologist  has 
to  work  out  the  details  for  his  own  neighbourhood,  just  as 


282  WEATHER. 

the  geologist  explains  the  local  peculiarities  of  his  own 
scenery  by  the  combination  of  general  and  local  causes. 
Local  is  also  like  diurnal  weather,  in  so  far  that  the 
observed  weather  is  the  sum  of  the  local  variations  and 
general  causes.  When  the  general  are  strong,  the  local 
are  entirely  masked;  when  the  general  are  weak,  then 
the  local  become  of  primary  importance.  We  shall  con- 
fine ourselves  to  a  few  examples  relating  to  cloud,  rain, 
and  hail,  so  as  to  exemplify  the  general  principles- 
involved. 


LOCAL  CLOUD. 

By  far  the  most  important  and  difficult  source  of 
local  variation  of  weather  is  found  in  the  development 
of  cloud,  rain,  and  other  forms  of  precipitation  by  the 
influence  of  seas,  lakes,  rivers,  hills,  and  valleys ;  some  of 
these  phenomena  are  so  interesting  that  we  propose  to 
devote  a  few  pages  to  their  consideration. 

We  will  commence  with  cloud,  though  we  must  re- 
member that  in  most  cases  cloud  is  only  undeveloped 
rain,  and  that  the  same  cause  which,  when  slight,  gives 
cloud  will  give  rain  when  more  intense.  In  our  diagram 
of  cyclone-weather  and  prognostics  (Fig.  2),  we  have 
marked  cumulus-cloud  in  rear  of  the  trough.  In  England 
this  holds  all  through  the  year,  but  in  the  drier  climate 
of  Continental  Europe  it  is  only  true  during  the  summer 
months.  The  reason  is  simply  that,  in  cold  climates, 
there  is  only  sufficient  vapour  to  develop  that  form  of 
cloud  in  summer.  What  we  have  specially  to  note  is,, 
that  when  cumulus  does  not  form,  it  is  not  replaced  by 


LOCAL  VARIATION   OF  WEATHER.  283 

any  other  kind  of  cloud,  but  the  sky  clears  without  any 
cloud  at  all.  For  instance,  no  local  variation  could  ever 
turn  the  cumulus  of  the  rear  into  the  cirrus-stratus  of  the 
front  of  a  cyclone ;  the  quantity,  not  the  quality,  could 
alone  be  changed.  Similarly  for  rain.  We  shall  see 
directly  that  the  actual  amount  may  vary  enormously 
from  local  causes,  but  no  peculiarity  can  turn  the  drizzling 
rain  of  a  cyclone-front  into  the  heavy,  big-dropped  shower 
of  a  thunderstorm,  or  vice  versa. 

A  very  striking  illustration  of  the  influence  of  local 
peculiarities  in  the  formation  of  cloud  was  once  observed 
by  M.  Flammarion  during  a  balloon-voyage  from  Paris  to 
the  Ehine.  He  saw  one  afternoon  that  cloud  was  formed 
over  the  rivers  and  woods,  but  not  over  the  open  plains. 
The  synoptic  charts  for  that  evening  show  that  Eastern 
France  was  then  covered  by  a  large  cyclone  of  moderate 
intensity ;  and  the  explanation  of  the  whole  is,  that  all 
the  air  in  that  part  of  Europe  was  then  in  a  rising  con- 
dition, but  that  it  was  only  over  rivers  and  damp  woods 
that  enough  vapour  was  present  to  condense  into  cloud. 
A  more  intense  cyclone  would  have  developed  cloud 
everywhere,  and  rain  only  over  the  rivers  and  forests ; 
another  still  more  intense  would  have  brought  rain  every- 
where. In  our  chapter  on  Prognostics,  we  alluded  to  mist 
being  formed  over  rivers  in  fine  frosty  weather.  Here, 
too,  we  have  local  variation,  but  of  a  contrary  nature  to 
that  which  we  have  just  considered.  This  example  will 
serve  to  call  attention  to  the  great  importance  of  dif- 
ferentiating between  the  various  kinds  of  condensed 
vapour. 

Another  very  common  local  cloud  is  that  which  rests 


284  WEATHER. 

on  or  over  hilltops,  when  blue  sky  covers  the  plains. 
This,  of  course,  is  due  to  the  horizontal  currents  of  the 
air  being  deflected  upwards,  and,  if  sufficient  vapour  is 
present,  cloud  is  formed  by  condensation.  The  most 
interesting  thing  about  these  clouds  is  that  they  remain 
stationary  as  a  whole,  though  their  outlines  and  con- 
stituent particles  are  in  constant  motion.  Their  prog- 
nostic value  has  been  already  explained  in  a  previous 
chapter. 

LOCAL  KAIN. 

We  shall  now  explain  a  few  of  the  principal  causes 
which  affect  the  quantity  of  rainfall.  One  of  the  com- 
monest and  most  obvious  is  that,  when  the  wind  which 
blows  over  water  first  meets  the  land,  rain  will  be  pre- 
cipitated. For  instance,  in  England,  with  a  cyclone  of 
moderate  intensity  and  a  westerly  wind,  rain  will  only 
fall  on  the  western  coasts  and  on  the  high  ground  inland. 
With  an  east  wind,  on  the  contrary,  the  fall  will  be 
confined  to  the  most  exposed  portions  of  the  east  coast, 
and  in  a  less  degree  to  high  inland  stations.  A  similar 
effect  is  found  all  over  the  world.  For  instance,  in 
Ceylon  the  rainy  seasons  on  the  two  sides  of  the  island 
are  in  different  months,  which  depend  on  the  time  when 
each  coast  is  exposed  to  the  prevailing  monsoon.  The 
south-west  monsoon  brings  rain  to  the  exposed  west  side 
of  the  island,  and  the  dry  season  to  the  east  coast,  which 
is  then  a  lee  shore.  The  north-east  monsoon,  on  the 
contrary,  first  strikes  the  east  coast,  and  develops  abundant 
rainfall  there;  while  the  west  coast  then  enjoys  its  dry 


LOCAL  VARIATION   OF  WEATHER.  285 

season.  A  similar  sequence  is  observed  on  the  opposite 
coasts  of  the  island  of  Luzon,  in  the  Philippines,  and  for  a 
similar  reason. 

But  though  the  sea  may  assist  in  the  development  of 
rain  under  suitable  circumstances,  the  presence  of  water 
alone  will  not  cause  rain.  The  rainless  districts  of  Peru 
and  Arabia  both  border  on  the  sea-coast,  but  no  rain  falls 
in  either  country.  In  the  former  the  persistent  anti- 
cyclone which  habitually  covers  that  country  does  not  form 
the  ascensional  current  necessary  for  rain,  though  the  air 
is  damp  enough  to  deposit  very  copious  dew;  in  the 
latter,  though  the  isobars  are  sometimes  curved  cycloni- 
cally,  the  rising  currents  never  seem  to  be  sufficiently 
strong  or  vapour-laden  to  produce  rain. 

Allied  to  the  influence  of  water  in  supplying  an 
abundant  source  of  vapour,  is  the  presence  of  a  damp 
surface,  such  as  a  thick  forest.  The  leaves  of  the  trees 
retain  so  much  moisture  that  the  air  is  always  damper 
over  wood  than  over  earth.  Then,  as  we  have  just 
explained  in  talking  about  the  development  of  cloud, 
when  an  ascensional  impulse  is  propagated  over  the 
country,  rain  will  sometimes  fall  over  the  woodlands 
when  it  would  not  be  precipitated  over  the  cultivated 
soil.  For  instance,  in  the  State  of  Iowa,  Dr.  Hinrichs  has 
shown  that  the  amount  of  mean  rainfall  is  very  materially 
influenced  by  the  position  of  the  timber-line,  and  numerous 
similar  cases  have  been  recorded  in  other  parts  of  the 
world. 

But  observations  on  this  point  are  very  discrepant. 
In  some  parts  of  Germany  the  influence  of  forests  is  said 
to  be  enormous;  in  India  the  effect  appears  to  be  less 


286  WEATHER. 

marked ;  while  the  Swedish  meteorologists  can  find  little 
relation  between  rainfall  and  the  covering  of  the  earth. 
The  facts  are  doubtless  as  they  have  been  described ;  and 
the  apparent  discordance  is  only  another  example  of  the 
great  principle  that  all  meteorological  results  are  the 
balance  of  various  circumstances.  The  difference  in 
humidity  over  trees  or  soil  will  be  much  less  in  a  cold 
country  like  Sweden,  than  under  a  blazing  Indian  sun ; 
and  a  great  deal  will  depend  on  whether  the  principal 
rainfall  is  induced  by  cyclones  or  secondaries. 

MOUNTAIN  BAIN. 

So  far  we  have  treated  of  water  and  forest  as  merely 
supplying  the  material  for  rain ;  now  we  must  consider  a 
little  more  in  detail  how  hills  and  valleys  will  affect  the 
precipitation  of  any  current.  In  England,  roughly  speak- 
ing, if  a  range  of  hills  under  about  fifteen  hundred  feet 
obstructs  the  prevailing  westerly  wind,  the  greatest 
amount  of  rain  will  fall  on  the  east  side  of  the  range. 
This  is  because,  though  the  moist  current  is  deflected 
upwards  on  the  west  side,  the  condensed  vapour  is  blown 
over  the  top  of  the  hill  and  falls  on  the  opposite  slope. 
If  the  range  is  over  fifteen  hundred  feet,  then  the  rain 
cannot  blow  over,  and  the  greatest  rain  will  fall  on  the 
west  side  of  the  hills.  No  rule  can,  however,  be  laid 
down  except  in  very  general  terms,  for  every  hill  and 
every  valley  has  its  own  local  peculiarities  in  the  manner 
in  which  it  develops  rain  with  different  winds.  Every 
country,  and  every  part  of  each  country,  must  be  worked 
out  in  detail  on  the  spot. 


LOCAL   VARIATION   OF   WEATHER.  287 

The  amount  which  will  be  deposited  at  different 
heights  will  also  vary  from  a  number  of  circumstances. 
For  instance,  on  the  west  coast  of  Scotland,  which  is  con- 
stantly exposed  to  south-west  winds,  the  rainfall  on  the 
low  western  islands  is  only  about  forty  inches  in  the  year ; 
while  along  the  watershed,  which  forms  the  backbone  of 
Scotland,  the  precipitation  exceeds  one  hundred  inches  in 
many  places.  In  Ceylon,  to  which  we  have  already 
alluded,  the  fall  on  some  coast  stations  does  not  rise  above 
thirty-four  inches,  while  some  of  the  mountain  stations 
record  no  less  than  two  hundred  and  nine  inches.  The 
difference  is  readily  accounted  for  when  we  consider  the 
relative  altitudes  of  the  respective  mountains  and  the 
greatly  increased  quantity  of  vapour  which  an  air-current 
of  90°  temperature  can  carry,  compared  with  one  of  only 
40°.  These  two  latter  numbers  represent  about  the  mean 
temperatures  of  the  two  countries ;  and  while  the  water- 
shed of  Scotland  rarely  rises  above  two  thousand  five 
hundred  feet,  many  of  the  mountains  of  Ceylon  attain  an 
altitude  of  six  thousand  feet. 


VALLEY  EAIN. 

It  is  this  property  of  mountains  in  developing  rain 
which  gives  truth  to  the  well-known  saying,  "  Hills  draw 
rain."  But  there  are  two  sources  of  rain  which  are 
intensified  by  valleys — the  rain  of  thunderstorms  and 
tidal  showers.  These  we  must  now  consider.  In  Great 
Britain  it  is  a  common  remark  that  thunder-showers  have 
a  tendency  to  run  along  the  course  of  rivers.  The  only 
-class  of  thunderstorm  which  does  not  follow  this  rule  is 


288  WEATHER. 

that  particular  kind  which  occurs  in  the  winter  months 
on  the  exposed  western  coast  of  Scotland  and  Ireland. 
These  are  certainly  thunder-squalls,  which  belong  to  large 
cyclones,  much  developed  by  mountains.  In  France  and 
other  countries  an  immense  amount  of  labour  has  been 
expended  in  tracking  thunderstorms,  as  we  have  already 
mentioned  in  a  preceding  chapter.  Though  the  storms 
as  a  whole  travel  in  a  north-easterly  direction  for  short 
distances,  the  course  of  rivers  is  found  to  exercise  a  very 
powerful  influence  both  on  their  path  and  still  more  on 
their  intensity.  Forests  and  hills  also  modify  the 
development  of  thunderstorms  to  a  less  extent,  so  that  we 
may  conveniently  consider  them  all  together. 

LOCALIZATION  OF  HAILSTORMS. 

• 

But  first  we  may  give  an  example  of  the  actual  facts. 
Since  hail  may  be  considered  as  the  most  intense  form  of 
a  thunderstorm,  we  have  given  in  Fig.  59  a  reduction  of 
a  chart  illustrating  the  distribution  of  hail  in  the  French 
Department  of  Loiret. 

The  river  Loire  will  be  readily  recognized  running 
across  the  diagram  from  right  to  left,  as  well  as  some  of 
its  smaller  tributaries.  A  well-known  conventional  symbol 
marks  the  limits  of  the  forest  of  Orleans,  while  the  small, 
round  points  indicate  the  number  of  years  in  which  any 
.commune  has  been  attacked  by  hail  during  the  thirty 
years  1836-1865.  The  scale  of  miles  shows  what  a  small 
area  we  have  to  deal  with ;  but  see  what  a  difference  in 
the  number  of  hailstorms.  In  the  town  of  Orleans,  011 
the  river  Loire,  sixteen  destructive  hailstorms  have  been 


LOCAL  VARIATION   Ofr  WEATHER. 


289 


recorded ;  and  at  the  village  of  Jangeau,  a  little  higher  up 
the  river,  twelve  serious  falls.  Yet  within  a  couple  of 
miles  of  both  places,  other  communes,  such  as  Clery,  have 
not  been  visited  by  more  than  two  or  three  storms.  Then 
observe  the  influence  of  the  mass  of  forest  to  the  north  of 


Cler 


FIG.  59. — Localization  of  hail  iu  Loiret.  The  points  indicate  the 
number  of  years  in  which  any  commune  has  been  attacked  by  hail 
during  the  thirty  years  1836-1865. 

the  river.     All  the  villages  to  the  west  of  the  forest  area 
have  a  large  number  of  points,  while  those  inside  the' 
forest  enjoy  almost  complete  immunity  from  destructive 
hailstorms. 


290  WEATHER. 

The  forest  seems  to  act,  in  fact,  as  a  breakwater,  against 
which  the  violence  of  the  storms  expends  itself  for  a  time, 
till  it  can  gather  fresh  force.  Almost  all  the  French 
observers  are  agreed  as  to  the  origin  of  this  development 
and  protection.  We  must  recall  the  fact  which  we 
mentioned  in  our  chapter  on  thunderstorms,  that  the  wind- 
circulation  of  thunderstorms  is  always  confined  to  the  low 
strata  of  the  atmosphere  only. 

Hail  is  produced  when  two  clouds  are  superimposed  at 
a  certain  distance.  A  storm  is  never  isolated.  The 
ordinary  French  ones  are  often  formed  by  partial  deriva- 
tions taken  from  the  south-west  winds,  and  occasioned  by 
the  passage  of  cyclones.  When  the  thick  mass  of  cloud 
which  marks  the  existence  of  a  storm  meets  a  valley,  the 
lower  clouds  are  diverted  from  the  general  route ;  a 
portion  follows  more  or  less  exactly  the  contours  of  the 
valley.  Thus  it  happens  that  the  clouds  passing  at  a 
great  height  from  the  ground  cross  those  which,  entangled 
in  the  valley,  have  been  deflected  by  the  successive  bends 
of  the  river  into  a  direction  different  from  the  south-west. 
It  is  then  that  hail  falls. 

In  the  same  way,  when  the  lower  clouds  meet  such  an 
•obstacle  as  a  forest,  or  even  a  mountain,  eddies  are  formed. 
The  masses  of  cloud  come  back  on  themselves,  and  seem 
to  be  repelled  and  dispersed  by  the  forest.  When  the 
clouds  have  succeeded  in  passing  the  obstacle,  their  force 
is  exhausted,  and  they  only  precipitate  rare  or  inoffensive 
hail,  and  do  not  regain  their  intensity  for  some  time 
afterwards. 

From  this  we  can  readily  understand  the  manner  in 
which  hills  and  valleys  develop  their  respective  rains. 


LOCAL  VARIATION   OF  WEATHER.  291 

In  a  large,  deep  cyclone,  hills  deflect  the  moist  currents 
upwards  on  a  large  scale,  and  the  greatest  rain  falls  in  the 
mountains.  In  shallow  secondaries,  with  slight  general 
wind-force,  the  rivers  and  forests  give  rise  to  local  eddies, 
which  for  some  reason  develop  the  precipitation  of  rain, 
and  especially  of  hail. 

TIDAL  SHOWERS. 

Tidal  showers  are  of  very  little  practical  importance, 
but  they  may  advantageously  be  mentioned  as  belonging 
to  the  class  of  rain  which  hugs  the  valleys,  and  not  the 
hills.  These  showers  are  so  called  because  they  are 
brought  up  by  the  tide,  either  along  the  coast  or  up  tidal 
rivers.  How  the  rising  water  should  develop  rain,  we 
cannot  explain,  but  the  character  of  the  influence  is  very 
obvious.  On  a  cloudy  day,  when  showers  or  heavy  masses 
of  vapour  are  flying  about,  it  is  frequently  observed  that 
after  the  tide  turns  to  rise,  and  the  stream  is  running 
upwards,  the  weather  begins  to  get  worse,  so  that  what 
was  merely  a  mass  of  cloud  before  will  now  precipitate 
rain.  This  rain  is  quite  local,  and  does  not  extend  far 
from  the  river-banks. 

In  calm  weather,  a  wind  also  often  comes  up  with  the 
tide,  or  if  the  flow  of  the  tide  assists  the  general  direction 
of  the  wind,  the  latter  will  be  much  increased  in  force 
and  gustiness.  If  the  day  is  really  fine,  of  course  the 
tide  will  not  bring  up  any  rain,  though  it  may  modify 
the  wind. 

From  this  description,  the  general  nature  of  tidal 
action  on  weather  will  be  sufficientlv  obvious.  For  some 


292;  WEATHER. 

reason  or  other  the  rise  of  the  tide  increases  the  intensity 
of  the  existing  system  of  weather.  If  this  is  tending 
towards  precipitation,  the  tide  will  give  just  the  last  impulse 
which  is  required,  and  rain  will  fall.  If  the  ascensional 
impulse  is  strong  enough  of  itself,  it  will  rain  independently 
of  the  tide ;  and  if  the  natural  impulse  is  downwards,  as 
in  an  anticyclone,  no  tide  is  sufficient  to  invert  the  general 
character  of  the  weather  and  cause  rain.  Tidal  influence 
on  weather  is  found  all  over  the  world.  Professor  Hazen 
has  found  a  marked  increase  of  thunderstorms  with  a 
rising  as  opposed  to  a  falling  tide  in  the  United  States  ; 
and  the  author  has  observed  a  well-defined  tidal  variation 
of  the  trade  wind  in  tropical  Fiji. 

We  can,  therefore,  sum  up  the  contents  of  this  chapter 
very  easily.  All  over  the  world  local  influences  modify, 
but  do  not  make,  the  general  character  of  the  weather. 
When  the  latter  is  weak,  local  weather  may  be  the 
prominent  feature  of  the  climate  of  any  place  ;  when  it  is 
strong,  then  local  influences  may  be  entirely  obliterated. 


(     293     ) 


CHAPTER  XL 

DIURNAL   VARIATION   OF   WEATHER. 

IN  this  chapter  we  propose  to  explain  how  to  collate  the 
variations  of  weather  that  are  found  in  many  places  to 
depend  on  the  hour  of  the  day,  with  the  great  principles 
of  the  relation  of  weather  to  the  distribution  of  atmo- 
spheric pressure  which  lie  at  the  bottom  of  all  modern 
meteorology.  In  many  places  the  direction  of  the  wind 
changes  reguiaiiy  at  certain  hours,  or  cloud  and  rain 
gather  at  the  same  time  day  after  day ;  how  can  all  this 
be  reconciled  with  the  laws  of  the  dependence  of  wind  on 
gradient,  and  of  weather  on  the  shape  of  isobars  ? 

The  cases  which  arise  in  practice  are  endless.  Every 
country,  every  season,  has  its  characteristic  diurnal 
weather,  and  a  complete  account  of  these  variations  in 
some  climates  would  more  than  fill  the  whole  of  this 
volume.  We  must,  therefore,  content  ourselves  with  a 
statement  of  the  general  principles  which  are  found  by 
observation  to  regulate  all  diurnal  variations,  and  with 
illustrations  of  a  few  typical  examples  from  various  parts 
of  the  globe. 


294  WEATHER. 

INDEPENDENCE  OF  DIURNAL  VARIATIONS  AND  GENERAL 
CHANGES. 

The  great  principle  which  underlies  all  diurnal 
weather  is  that  diurnal  variation  modifies  but  never 
alters  the  general  character  of  the  weather,  which  is 
determined  by  the  distribution  of  surrounding  pressure. 
In  England  the  amount  of  cloud  or  rain  in  a  cyclone 
will  vary  at  different  hours,  but  the  kind  of  cloud  and 
quality  of  the  rain  will  never  be  altered.  In  like  manner, 
the  land  and  sea  breezes  at  Bombay  will  veer  or  back 
with  the  sun,  but  the  general  character  of  the  wind  due 
to  the  monsoon  of  the  season  will  never  be  lost.  This  law 
of  weather  not  only  enables  us  to  explain  many  phenomena 
of  weather  which  would  otherwise  present  a  chaos  of 
discordant  observations ;  but  it  also  serves  to  guide  our 
research  into  the  great  problems  of  weather- forecasting. 
When  once  we  know  that  we  may  safely  neglect  all  con- 
siderations of  diurnal  variations  when  we  wish  to  study 
the  motion  of  depressions  and  the  consequent  changes 
of  weather,  our  task  is  thereby  much  simplified.  If  we 
were  to  discuss  the  statistical  values  of  meteorological 
elements,  we  should  find  that  all  the  voluminous  results 
which  have  been  obtained  by  the  method  of  averages  are 
of  no  use  in  forecasting,  and  that  the  diurnal,  values  of 
wind  or  rain  which  have  thus  been  obtained  have  nothing 
to  do  with  weather-changes. 

DIURNAL  TEMPERATURE. 

In  this  independence  of  the  general  changes  of 
weather,  diurnal  are  very  like  local  variations ;  but  there 


DIURNAL  VARIATION  OF  WEATHER.  295 

is  one  important  difference — that  diurnal  variations  intro- 
duce us  for  the  first  time  in  this  book  to  the  consideration 
of  the  true  nature  of  all  meteorological  periodicities. 
From  the  variations  which  run  through  their  entire 
course  in  one  day,  we  can  readily  pass  to  those  whose 
period  is  one  year,  or  even  a  longer  cycle. 

We  shall  therefore  commence  with  an  account  of  the 
nature  of  the  diurnal  variations  of  heat,  as  that  is  the 
most  obvious  of  all  meteorological  phenomena.  It  is 
evident  to  all  the  world  that,  whatever  the  temperature 
of  the  day  or  season  may  be,  the  nights  are  in  a  general 
way  colder  than  the  clays ;  this  is  the  ordinary  instinctive 
idea  of  diurnal  variation.  We  also  know  almost  by 
instinct  whether  it  is  a  generally  cold  or  hot  day ;  and 
equally  instinctively  we  allow  for  the  difference  of  day 
and  night.  Put  into  the  formal  language  of  ordinary 
meteorology,  the  general  heat  or  cold  of  the  day  is  ex- 
pressed by  the  number  which  gives  the  mean  temperature 
of  the  day ;  while  the  diurnal  variation  or  range  is  given 
by  the  numbers  which  denote  how  much  the  thermometer 
was  above  or  below  the  average  at  each  hour.  In  a 
climate  like  that  of  England,  most  people  have  a  vague 
idea  that  sometimes  in  winter  the  weather  gets  warmer  in 
the  evening,  after  a  white  frost  in  the  morning ;  but 
when  we  come  to  examine  self-recording  thermograms, 
we  find  that  irregular  changes  of  temperature  are  much 
more  common  than  is  usually  supposed.  W"e  have  already 
alluded  to  this  subject  in  our  chapter  on  Meteograms,  but 
we  give  here,  in  Fig.  60,  the  thermogram  at  Kew, 
December  7  to  9,  1874 — that  is,  for  the  same  period  as 
the  meteogram  and  charts  in  Figs.  25,  26,  and  27 ;  and 


296 


WEATHER. 


in  addition  to  this  we  give  in  Fig.  61  the  mean  annual 
curve  of  diurnal  variation  of  temperature  at  Kew,  as 
deduced  by  Mr.  Eaton.  The  point  which  most  concerns 

i   I  j   I    i    I 


5°  — 


jo— 


Dec 


TT 


1874 


Noon. 

I    I     I     I     I     i     1 


FIG.  60. — Thermograms. 

us  here  is  to  understand  exactly  what  the  significance  of 
the  curve  of  mean  temperature  is.  The  thermographic 
trace  for  three  days  at  Kew,  in  Fig.  60,  shows  very  fairly 
(«  Noon  f~  Qo  what  temperature-changes  are  really 
like  from  day  to  day.  In  spite  of 
endless  irregularities,  there  is  a  gene- 
ral tendency  in  the  first  two  days  for 
the  hottest  time  of  the  day  to  occur 
in  the  afternoon,  and  the  coolest  in 
the  early  morning;  while  on  the  third 
day  there  is  little  trace  of  diurnal 
,  variation,  but  a  steady  general  fall 

FIG.  61. — Mean  diurnal  _  J    ' 

range  of  temperature,    of  temperature  due  to  general  causes. 

When  the  mean  temperature  for  every 

hour  is  taken  on  a  great  many  days,  the  irregularities 

balance  out,  and  the  daily  tendency  only  is  reflected  in  the 

mean  curve  of  diurnal  range. 

If  we  lived  in  a  vapourless  climate,  the  sun  would 


DIURNAL  VARIATION   OF  WEATHER.  297 

impose  every  day  a  similar  record  on  the  thermograph, 
only  varying  a  little  in  amount  according  to  the  season. 
But  in  practice  the  sun  has  a  daily  struggle  with  the 
wind  and  cloud.  Some  days  a  shift  of  wind  to  the  south 
in  the  afternoon  will  cause  the  thermometer  to  rise 
steadily  as  the  sun  goes  down,  and  midnight  may  be  hotter 
than  noon;  on  other  days  a  dense  layer  of  mist  will 
completely  shut  off  the  influence  of  the  sun's  rays,  and 
the  instrument  will  leave  a  straight  horizontal  line  as  its 
record  of  temperature-variation  for  twenty-four  hours. 

All  that  the  mean  curve  of  temperature  signifies  is, 
;that  in  a  general  way,  allowing  for  all  sorts  of  irregu- 
larities, there  is  a  diurnal  solar  influence,  which  has  its 
greatest  and  least  values  at  such  and  such  hours.  But  we 
must  most  carefully  avoid  two  conclusions:  first,  that 
the  mean  temperature  represents  any  abstract  entity, 
called  mean  diurnal  range,  which  might  be  applied  as  a 
correction  to  the  observed  temperature  at  any  hour  so  as 
to  deduce  the  mean  temperature  of  the  day  ;  and,  second, 
that  because  we  do  not  see  a  diurnal  variation  on  the 
trace  for  every  day,  therefore  there  is  no  such  thing 
as  solar  diurnal  influence.  Because  the  thermogram  for 
December  9  (Fig.  60)  shows  no  diurnal  maximum  and 
minimum,  that  does  not  prove  that  there  is  no  such  thing 
as  diurnal  variation  at  all.  The  importance  of  this  last 
conclusion  will  be  evident  in  our  next  chapter  on  Cyclical 
Variations  of  Weather. 

What  more  immediately  concerns  us  now  is  to  note 
how  diurnal  affects  general  heat.  This  can  be  better 
accomplished  by  trying  to  recollect  the  history  of  any 
particular  day's  weather  than  by  the  inspection  of  dia- 


298  WEATHER. 

grams.  If  we  think  of  any  day,  we  shall  remember  that 
if  the  wind  did  not  shift,  the  general  character  of  the  heat 
did  not  alter.  Suppose,  for  instance,  that  we  had  been  in 
a  large  cyclone  with  a  north-west  wind,  which  lasted  for 
the  whole  twenty-four  hours.  We  should  have  known 
instinctively  that  it  was  a  cold  day  for  the  season,  and 
though  there  would  have  been  a  very  considerable  differ- 
ence between  the  temperatures  recorded  by  night  and  by 
day,  the  quality  of  heat  which  belongs  to  north-west 
winds  would  have  remained  the  same.  But  suppose  that, 
after  a  sharp  white  frost  in  the  morning,  about  midday 
the  sky  had  begun  to  thicken  and  the  wind  to  back  to 
the  south,  then,  as  before  mentioned,  the  temperature 
would  rise  while  the  sun  was  going  down,  but  every  one 
would  have  recognized  that  the  quality  of  the  heat  and 
the  general  character  of  the  weather  had  changed.  We 
thus  see  the  correctness  of  the  phraseology  which  calls 
such  changes  general,  which  have  their  origin  in  the 
great  movements  of  atmospheric  pressure  with  wind- 
shifts,  and  those  changes  variations  which  are  imposed 
on  the  general  character  by  the  sun's  daily  influence.  In 
fact,  we  realize  the  great  principle  that  the  diurnal 
variations  are  superimposed  on  the  general  changes,  but 
never  alter  the  character  of  the  latter.  Temperature, 
like  every  other  element  of  weather,  is  the  balance  of  the 
general  changes  and  diurnal  variations;  so  that  when 
the  general  are  strong  the  diurnal  are  masked,  when  the 
former  are  weak  the  latter  are  predominant. 


DIURNAL  VARIATION  OF  WEATHER.  299 

DIURNAL  CLOUD. 

From  the  comparatively  simple  nature  of  diurnal  heat, 
we  must  now  turn  to  the  far  more  complicated  variations 
of  fog,  cloud,  and  rain.  The  simplest  case  of  the  diurnal 
precipitation  of  vapour  is  found  in  the  regular  formation 
of  valley  or  river  mist  in  fine  weather.  In  settled  climates, 
at  certain  seasons,  the  sky  is  always  blue  by  day,  but  after 
dark,  fog  or  mist  begin  to  form  in  the  low-lying  ground, 
from  the  influence  of  nocturnal  radiation.  By  sunrise 
the  valleys  will  be  filled  with  mist,  which  rises  to  such  a 
uniform  level  that,  viewed  from  a  height,  the  hollows 
look  as  if  they  were  filled  with  water.  After  the  sun  is 
up,  the  vapour  gradually  rises  and  disperses,  till  the  sky 
resumes  its  usual  blue  appearance.  The  general  anti- 
cyclonic  character  of  the  weather  is  the  same  throughout, 
but  the  day  impresses  a  variation  on  the  face  of  the  sky. 

Diurnal  weather-variations  are  so  intricate,  and  vary 
so  much,  not  only  in  different  countries,  but  at  different 
seasons,  that  we  can  only  give  a  few  illustrations  of 
general  principles.  Every  one  of  the  seven  fundamental 
shapes  of  isobars  has  a  type  of  diurnal  weather  peculiar 
to  itself.  Only  that  of  the  two  great  shapes  has  been 
worked  out  for  Great  Britain  by  the  author.*  He  has 
shown  that  in  their  diurnal  variations  cyclones  and  anti- 
cyclones present  the  same  antithesis  as  they  do  in  all 
their  other  special  characteristics.  The  broadest  features 
of  the  diurnal  variation  in  a  cyclone  is  that,  starting 
from  the  early  morning,  the  amount  of  cloud  and  the 

*  Quarterly  Journal  of  the  Meteorological  Society,  London,  vol.  iv. 
p.  4. 


300  WEATHER. 

general  severity  of  the  weather  gradually  increase  till 
about  2  p.m.,  and  then  gradually  decrease  till  past  mid- 
night. In  anticyclones,  on  the  contrary,  a  misty  or  cloudy 
morning  is  followed  by  a  great  diminution  of  cloud  as  the 
day  goes  on,  while  later  on,  in  the  evening,  mist  and 
cloud  are  sometimes  formed  again. 

If,  besides  these  broad  features  of  diurnal  variation, 
we  consider  some  of  its  more  minute  changes,  we  observe 
that,  in  cyclones,  a  cloudy  morning  often  has  a  short 
break  about  10  a.m. ;  that  the  weather  then  becomes 
much  worse,  but  has  a  marked  tendency  to  clear  up  again 
about  4  p.m.  In  anticyclones,  on  the  contrary,  a  clear 
morning  at  4  a.m.  is  frequently  very  cloudy  at  10  a.m., 
after  which  the  cloud  again  decreases  till  about  4  p.m., 
when  more  cloud  or  mist  are  often  again  formed,  and  last 
till  quite  late  in  the  evening.  We  find,  in  fact,  not  only 
traces  of  a  semi-diurnal  variation,  or  of  one  which  runs 
through  its  course  twice  in  a  day,  but  also  that  the 
intervals  of  this  variation  are  obviously  connected  with 
the  hours  of  diurnal  maxima  and  minima  of  pressure. 

The  increase  of  cloud  during  the  day  in  cyclones  may 
be  generally  described  as  an  accession  of  intensity  which 
accompanies  the  diurnal  increase  in  the  velocity  of  the 
incurving  wind.  Then,  as  more  air  is  being  poured  into 
the  centre  of  the  cyclone,  the  ascensional  currents  must 
also  be  stronger,  and  therefore  more  cloud  will  be  formed. 
In  anticyclones,  the  morning  mist  has  already  been  shown 
to  be  due  to  radiation,  and  the  marked  clearing  of  the 
sky  during  the  day  must  be  due  to  the  increased  strength 
of  the  descensional  currents  near  the  centre,  caused  by 
the  increased  velocity  of  the  outcurved  wind  during  the 


DIURNAL   VARIATION   OF   WEATHER.  301 

day.  From  this  we  can  easily  understand  how  two 
different  shapes  of  isobars  can  have  such  very  different 
diurnal  variations.  In  one,  the  influence  of  the  sun 
modifies  a  rising  current ;  in  the  other,  a  descending  one. 
But  whatever  modification  the  diurnal  variation  may 
impose  on  a  cyclone  or  anticyclone,  the  general  character 
of  either  is  never  lost.  No  diurnal  variation  can  make 
the  cirrus  clouds  in  front  of  a  cyclone  either  like  the 
cumulus  in  rear,  or  like  the  clouds  formed  from  the  rising 
mist  of  an  anticyclone.  The  variation  is,  as  it  were,  a 
modifying  influence  superimposed  on  the  more  general 
features. 


DIURNAL  BAIN. 

The  diurnal  variation  of  rain  is  one  of  the  most 
difficult  questions  in  meteorology.  Not  only  does  the 
variation  differ  in  each  shape  of  isobars,  but  there  is  a 
tendency  of  small  secondaries  to  form  at  particular  hours ; 
and  moreover,  during  the  winter  months,  there  is  a  marked 
tendency  of  large  cyclones  to  come  in  from  the  Atlantic 
with  increased  intensity  during  the  night.  Besides  a 
diurnal  variation  of  the  intensity  of  a  rainy  shape*  of 
isobars,  there  is  a  diurnal  period  both  of  the  formation 
and  motion  of  cyclones. 

The  error  which  we  have  most  carefully  to  avoid  in 
treating  this  branch  of  the  subject,  is  the  supposition  that 
all  rain  is  cyclonic,  or  that  the  diurnal  period  of  thunder- 
storms and  secondaries  is  the  same  in  every  country.  From 
this  it  is  evident  that  if  one  shape  of  pressure-distribution 
predominates  in  summer,  and  another  in  winter,  then  the 


302  WEATHER. 

two  seasons  will  have  a  totally  different  type  of  diurnal 
rain-variation. 

We  can  get  a  most  striking  illustration  of  these 
principles  from  the  tropics.  At  Calcutta  Mr.  H.  F. 
Blanford  finds*  that  the  weather  is  divided  into  three 
seasons : — 

The  rains,  June  to  October,  when  the  diurnal  frequency 
curve  of  rain  begins  to  rise  soon  after  midnight  to  a  small 
maximum  about  6  a.m.  and  a  small  minimum  about  8  a.m. 
Then  the  frequency  rises  rapidly  to  its  principal  maximum 
at  2  p.m.,  and  falls  quickly  to  the  principal  minimum  at 
1  a.m.  The  mode  of  the  formation  of  the  rain-cloud  of 
the  summer  monsoon  is  essentially  cumulus. 

The  hot  season,  March  to  May,  when  the  diurnal 
epoch  of  minimum  is  not  very  distinctly  indicated,  but 
would  appear  to  occur  about  sunrise.  There  is,  however, 
little  variation  from  midnight  up  to  9  or  10  a.m. ;  and 
after  this,  only  a  slow  rise  up  to  2  p.m.,  when  the  increase 
becomes  more  rapid.  About  two  hours  before  sunset, 
there  is  a  sudden  rise  of  about  fifty  per  cent.,  and  the 
hour  of  maximum  raininess  occurs  between  7  and  8  p.m. 
Compared,  however,  with  the  maximum  of  the  rainy 
sea'son,  it  is  very  small.  This  very  striking  feature  of  the 
hot  season  is  due  to  evening  storms,  known  as  north- 
westers, which  are  so  called  because  they  commonly 
originate  in  the  north-west,  and  are  probably  connected 
with  the  diurnal  variation  of  wind  near  the  coast. 

Lastly,  the  cold  season  from  November  to  February. 
In  this,  falls  of  rain  are  pretty  evenly  distributed  through- 
out the  day,  with  a  decided  diminution  during  the  two  or 
*  Asiatic  Society,  Bengal,  xlviii,,  part  ii.,  1879. 


DIURNAL   VARIATION   OF   WEATHER.  303 

three  hours  before  and  after  midnight.  These  seasons 
are,  of  course,  associated  with  different  types  of  pressure- 
distribution.  The  rainy,  cold,  and  hot  seasons  belong  to 
the  periods  of  the  south-west  and  north-east  monsoons, 
with  an  intermediate  period  respectively.  When  these 
are  all  combined  into  a  yearly  curve,  the  result  is  a  curve 
which  gives  the  true  variation  of  no  season.  In  this  case, 
as  the  rainy  season  curve  is  very  pronounced,  and  that  of 
the  two  other  seasons  much  less  marked,  the  annual  curve 
differs  but  little  from  that  of  the  rainy  season,  though 
some  of  the  minor  flexures  are  altered.  Under  other 
conditions  the  mean  annual  curve  might  be  very  different 
from  that  of  any  one  season. 

In  some  countries,  where  land  and  sea  breezes  are 
tolerably  constant,  rain  often  falls  at  the  turn  of  the  wind, 
but  the  details  vary  indefinitely ;  and  in  many  parts  of 
the  tropics,  inland  as  well  as  on  the  sea-coast,  thunder- 
storms form  regularly  at  the  same  hour  every  day.  In- 
land this  does  not  usually  occur  before  2  p.m.,  and  usually 
later ;  but  no  rule  can  be  laid  down.  These  storms  are, 
of  course,  totally  non-isobaric. 

All  over  England  the  mean  diurnal  curves  of  rain  are 
so  irregular  that  they  do  not  show  any  real  variation ; 
for  there  are  so  many  kinds  of  rain  in  that  country — each 
with  its  own  variation — that  the  curve  of  all  mixed  up 
together  has  no  physical  significance  at  all.  In  Prague, 
Professor  Augustin  finds  three  types  of  diurnal  frequency 
for  winter,  summer,  spring  and  autumn,  respectively; 
while  in  many  parts  of  Europe,  and  in  Japan,  there  is  a 
tendency  to  develop  morning  and  evening  maxima  of 
rain,  with  day  and  night  minima.  Every  country  has  its 


304  WEATHER. 

own  peculiarities ;  and  each  set  must  be  explained  on  its 
own  merits. 

In  our  detail  of  the  cyclonic  variation  of  cloud,  in 
England,  we  pointed  out  that  the  diurnal  variation  does 
not  alter  the  general  character  of  the  sky  or  cloud,  or  we 
might  say  of  the  weather  as  a  whole..  The  same  great 
principle  holds  for  every  other  shape  of  isobars  and  for 
every  other  climate.  The  diurnal  variation  of  rain  in 
Calcutta  during  the  rainy  season  is  enormous,  but  at  all 
hours  the  general  character  of  the  south-west  monsoon  is 
always  the  same.  Similarly,  during  the  cold  weather  of 
the  north-east  monsoon  the  diurnal  variation  is  only  a 
modification,  not  a  real  change  of  weather. 

DIURNAL  WIND. 

In  our  chapter  on  Meteograms  we  have  partially 
explained  the  general  idea  of  diurnal  wind-variation,  but 
we  now  wish  to  give  some  additional  developments  of  the 
subject, 

DIURKAL  VELOCITY. 

The  commonest  feature  of  diurnal  wind  in  most  places, 
temperate  as  well  as  tropical,  is  an  increase  of  the  velocity 
from  daybreak  to  about  2  p.m.,  and  then  a  decrease  to 
its  lowest  point  about  4  a.m.  Besides  this,  there  is  in 
many  places  a  smaller  series  of  variations  whose  turning 
points  occur  about  the  same  time  as  the  maxima  and 
minima  of  diurnal  pressure.  For  instance,  in  Great 
Britain,  the  principal  maximum  is  about  2  p.m.,  and  the 
principal  minimum  about  4  a.m.  But  in  addition  to  this, 


DIURNAL   VARIATION   OF   WEATHER.  305 

there  is  a  small  minimum  about  10  p.m.,  with  a  small 
maximum  between  that  hour  and  the  great  minimum  at 
4  a.m.,  besides  a  well-marked  minimum  about  10  a.m., 
just  at  the  hour  when  cyclones  and  anticyclones  develop 
their  most  characteristic  difference  of  diurnal  formation 
of  cloud.  A  similar  tendency  is  found  all  over  the  world, 
both  north  and  south  of  the  equator.  The  details  vary 
indifferently,  and  cannot  be  said  to  have  been  fully  worked 
out  in  any  one  country. 

We  can,  however,  safely  say  that  the  variations  are 
all  of  the  diurnal  type,  and  have  nothing  to  do  with  the 
character  or  amount  of  wind- velocity,  which  depends  on 
the  distribution  of  surrounding  pressure.  Just  as  with 
weather,  the  general  character  of  wind  and  the  relation  to 
gradient  depend  primarily  on  the  isobars;  the  diurnal 
variation/'  in  spite  of  its  great  complexity,  is  purely 
secondary. 

Buchan  has  made  the  very  important  observation  that 
the  diurnal  variation  is  almost  nothing  over  the  sea,  when 
away  from  the  influence  of  land,  and  he  has  also  connected 
this  with  the  fact  that  the  diurnal  variation  of  temperature 
is  very  small  over  the  sea  compared  with  that  over  land, 
so  that  in  some  way  the  diurnal  amount  of  wind- velocity 
appears  to  depend  on  the  temperature  of  the  floor  over 
which  it  blows. 

Many  of  the  details  of  wind-variation  are  both  interest- 
ing and  puzzling.  In  some  places  the  wind  falls  about 
noon,  probably  from  some  local  influence ;  and  in  Great 
Britain,  Ley  has  shown  that  the  diurnal  variation  of 
velocity  is  greater  with  west  than  with  east  winds.  This 
again  coincides  with  an  observation  of  Hamberg's,  that 

x 


306  WEATHER. 

the  stronger  the  wind,  the  greater  the  amount  of  diurnal 
increase  of  velocity,  because  as  a  rule  in  England  west 
winds  have  a  higher  velocity  than  east  ones. 

So  far  we  have  only  thought  of  the  wind  on  the  earth's 
surface,  but  on  high  mountain  peaks  the  variation  of  wind 
velocity  is  almost  exactly  the  opposite ;  for  there  the 
maximum  is  in  the  early  morning,  and  the  minimum  about 
or  just  after  noon.  The  general  speed  of  the  wind  is, 
however,  much  greater  at  high  altitudes  than  at  low 
levels.  This  principle  appears  to  hold  equally  in  both 
hemispheres. 

DIURNAL  DIRECTION. 

The  diurnal  variation  of  direction  is  less  marked  than 
that  of  velocity  and  much  more  difficult  to  detect.  In 
the  northern  hemisphere,  however,  there  is  a  well-defined 
tendency  to  veer  a  little  in  the  morning,  and  to  back 
again  in  the  afternoon ;  so  that  the  times  of  greatest  veer- 
ing and  backing  .correspond  to  the  hours  of  greatest  and 
least  velocity.  That  is  to  say,  if  the  general  direction 
of  the  wind,  is  from  the  west,  it  will  be  a  little  more  from 
the  north-west  by  day,  and  a  little  more  from  the  south- 
west by  night ;  the  greatest  northing  being  about  2  p.m., 
and  the  greatest  southing  about  4  a.m.  There  are  also 
traces  of  a  semi-diurnal  variation  exactly  similar  to  that 
which  we  described  under  the  head  of  velocity;  but  we 
cannot  give  complete  details  for  any  one  place. 

On  mountain-tops  the  daily  oscillation  of  the  wind  is 
on  a  contrary  system,  for  there  the  wind  backs  by  day 
and  veers  towards  evening.  For  instance,  a  generally 


DIURNAL   VARIATION   OF  WEATHER.  307 

westerly  wind  will  back  towards  west  by  south  till  the 
afternoon,  and  then  veer  towards  west  by  north  at  night- 
fall. We  demonstrated  in  our  chapter  on  Clouds,  under 
the  heading  of  "Vertical  Succession  of  Air-Currents," 
that  in  the  northern  hemisphere  successive  vertical  strata 
of  wind  came  more  and  more  from  the  left  of  an  observer 
standing  with  his  back  to  the  wind  on  the  surface.  If, 
then,  the  surface  current  veers,  and  the  upper  ones  back 
during  the  day,  the  result  will  be  that,  both  in  cyclones 
and  anticyclones,  the  difference  in  direction  between  the 
upper  and  lower  currents  will  be  greater  by  night  than 
by  day. 

In  the  southern  hemisphere  we  have  not  a  sufficient 
number  of  observations  to  enable  us  to  generalize  on  the 
nature  of  diurnal  wind-variation  ;  but  as  far  as  they  go 
they  point  to  an  exactly  similar  law  to  that  which  holds 
in  the  northern  hemisphere.  The  surface  winds  veer, 
and  the  upper  currents  back  with  the  course  of  the  sun. 
But  observe  that  the  course  of  the  sun  is  opposite  in  the 
two  hemispheres,  so  that  a  westerly  surface  wind  would 
veer  towards  north  in  the  northern  hemisphere,  and 
towards  south  in  the  southern  hemisphere.  As  some 
may  prefer  to  see  the  laws  of  diurnal  wind  exhibited  in 
their  relation  to  absolute  direction,  as  given  by  the  hands 
of  a  watch,  we  may  state  these  results  thus — 

FORENOOX  AFTERXOOX. 

Northern  hemisphere — Surface  ...  With  watch-hands  ...  Against. 

„  „  Hilltops  ...  Against       „          ...  With. 

Southern  hemisphere — Surface  ...  Against        „  ...  With. 

„  „  Hilltops  ...  With  „  ...  Against. 

The  following  is  a  fair  illustration  of  the  nature  and 
amount  of  diurnal  wind-variation,  both  of  velocity  and 


308 


WEATHER. 


direction,  as  ordinarily  observed  in  Great  Britain.  In 
Fig.  62  we  give  a  copy  of  the  anemographic  record  at 
Kew,  near  London,  for  the  three  days,  August  6th  to  8th, 
1874. 


DtrcctuTrv 


KEW 


Noon  Nooiv  JSoow 

FIG.  62. — Anemographic  curves  for  Kew,  August  6th  to  8th,  1874. 

The  synoptic  conditions  for  these  three  days  were  the 
commonest  in  that  country.  A  series  of  large  cyclones  of 
moderate  intensity  were  passing  to  the  north  of  Great 
Britain,  so  that,  although  there  was  a  good  deal  of  cloud 
and  wind,  there  was  not  the  marked  shift  of  wind  which 
would  occur  if  the  cyclone  centres  had  passed  nearer  the 
station.  Taking  the  velocity  first,  in  the  first  two  days 
the  tendency  of  the  wind  to  rise  during  the  day  is  very 
obvious ;  but  on  the  third  day  the  ordinary  variation  is 
completely  masked  by  violent  squalls.  Two  of  these 
occurred  at  the  times  marked  q  on  the  diagram.  The 
smaller  semi-diurnal  variations  are  also  almost  completely 
obliterated ;  they  are  only  observed  in  calm,  summer, 
anticyclonic  weather. 

Then,  looking   at    the   direction-traces,   the   general 


DIUKNAL  VARIATION  OF    WEATHER.  309 

westerly  direction  of  the  wind  due  to  the  isobars  is 
sufficiently  obvious,  but  superimposed  on  that  we  find 
every  day  an  irregular  tendency  for  the  wind  to  veer  a 
little  towards  the  north-west  during  the  day,  and  to  back 
again  during  the  night.  We  also  see  another  feature  of 
British  winds,  viz.  the  increased  gustiness  by  day  com- 
pared with  the  night.  This  is  shown  by  the  more 
irregular  trace  during  the  day  hours. 

From  a  simple  case  of  this  sort  we  can  readily  see 
both  the  amount  of  diurnal  variation  as  well  as  the 
manner  in  which  it  can  easily  be  masked.  When  the 
general  features  of  the  weather  are  feebly  marked,  then 
the  diurnal  variations  are  strong,  and  may  be  the 
prominent  character  of  the  day.  This  is  common  in 
many  tropical  countries,  especially  in  those  which  are 
habitually  covered  by  anticyclones.  In  variable  climates, 
like  that  of  Great  Britain,  on  the  contrary,  the  diurnal 
variations  are  only  obvious  in  the  finest  settled  summer 
weather ;  and  in  winter,  when  the  general  changes  are 
very  intense,  the  diurnal  features  are  often  completely  lost. 

We  may  then  lay  down  as  a  general  rule  that  the 
prominence  of  the  diurnal  variations  of  weather  are  a 
measure  of  the  settled  or  unsettled  character  of  the 
climate  of  any  place. 

Any  attempt  to  discuss  all  the  details  of  diurnal  wind, 
or  the  different  theories  which  have  been  suggested,  with 
more  or  less  probability,  to  account  for  it.  is  beyond  the 
scope  of  this  work.  All  that  we  wish  to  do  is  to  give  a 
clear  sketch  of  the  general  nature  of  diurnal  variation, 
and  especially  of  the  manner  in  which  it  is  subordinated 
to  the  great  laws  of  dependence  of  weather  on  isobars. 


310  WEATHER. 

GENERAL  VIEW  OF  THE  SUBJECT. 

It  is  most  interesting  to  note  the  unity  which  runs 
through  the  whole  class  of  diurnal  variations.  The 
principal  maxima  and  minima  of  temperature,  wind,  and 
partially  weather  at  2  p.m.  and  4  a.m.  respectively,  are 
strictly  analogous  to  each  other,  while  the  semi-diurnal 
features  of  wind  and  weather  are  analogous  to  the  diurnal 
variation  of  pressure.  The  latter,  to  which  we  have 
scarcely  alluded,  has  two  minima  at  4  a.m.  and  4  p.m., 
and  two  maxima  at  10  a.m.  and  10  p.m.  respectively.  The 
single  diurnal  variations  are  undoubtedly  due  to  the  direct 
influence  of  the  sun's  heat ;  but  the  question  how  an 
influence  such  as  that  which  runs  its  course  only  once  in 
the  twenty-four  hours  can  induce  a  variation  which  has  a 
semi-diurnal  period,  has,  up  to  the  present  time,  baffled 
the  skill  of  meteorologists.  It  is,  however,  perfectly  certain 
that  no  one  is  the  cause  of  the  others ;  all  are  equally 
the  products  of  the  same  influence,  and  no  comprehensive 
theory  of  diurnal  variations  will  ever  be  complete  which 
does  not  explain  and  co-relate  all  together.  When  we 
look  at  a  series  of  synoptic  charts  for  several  consecutive 
days,  we  see  that  many  cyclones  go  on  their  course  often 
for  two  or  three  weeks,  quite  independent  of  diurnal 
changes.  We  may,  therefore,  perhaps  suggest  the  follow- 
ing broad  view  of  the  relation  of  diurnal  variation  to  the 
general  character  of  weather.  The  whole  atmosphere  is 
circulating  between  the  equator  and  the  poles.  Some- 
times this  flow  of  air  takes  the  form  of  eddy  known  as  a 
cyclone,  sometimes  that  known  as  an  anticyclone,  and 
almost  always  one  of  the  seven  fundamental  forms  of 


DIURNAL  VARIATION   OF   WEATHER.  311 

circulation.  Every  day,  as  the  sun  rises  and  sets  on  this 
system,  he  impresses  either  directly  or  indirectly  a  series 
of  complex  variations  on  every  meteorological  element, 
but  does  not  change  the  intrinsic  nature  of  any  form  of 
circulation. 

The  results  of  this  chapter  may  therefore  be  summed 
up  as  follows.  In  every  part  of  the  world  the  diurnal 
variation  is  superimposed  on  the  general  character  of  the 
weather,  which  is  due  to  the  distribution  of  surrounding 
pressure.  The  resulting  weather  is  the  balance  of  the 
general  character  and  diurnal  variation ;  the  prominence 
of  the  diurnal  is  a  measure  of  the  settled  nature  of  the 
climate  of  any  place. 

All  over  the  world  there  is  a  tendency  to  form  both  a 
single  diurnal  variation,  which  varies  only  once  in  the 
twenty-four  hours,  and  a  semi-diurnal  variation,  which 
has  two  maxima  and  two  minima  in  the  same  time.  The 
origin  of  the  first  is  undoubtedly  the  direct  action  of  the 
sun ;  that  of  the  latter  cannot  be  at  present  explained. 
No  diurnal  variation  has  any  effect  on  general  weather, 
and  can  be  neglected  in  all  questions  which  relate  to 
forecasting  general  changes.  This  independence  is  one 
of  the  most  important  principles  in  meteorology. 


312  WEATHER. 


CHAPTEE  XII. 
ANNUAL  AND  SECULAR  VARIATIONS. 

SEASONAL  APPEARANCE  OF  THE  SKY. 

THE  term  "  seasonal  variation  "  is  used  in  a  twofold  sense. 
In  the  simpler  case,  it  refers  to  the  minute  differences  in 
the  appearance  of  the  sky  which  are  found  at  various 
seasons  in  cyclones,  etc.  For  instance,  the  rear  of  a 
cyclone  does  not  form  cumulus  cloud  in  the  dry  winter 
months  of  Continental  Europe ;  only  blue  sky  is  seen.  In 
damp  England,  cumulus  is  formed  at  all  seasons ;  but  is 
much  denser  and  more  strongly  marked  in  summer  than 
in  winter.  In  like  manner,  a  secondary  which  would 
develop  thunder  in  summer  in  Great  Britain  would  only 
produce  heavy  rain  in  winter.  In  this  way  seasonal  is 
exactly  analogous  to  diurnal  variation,  for  it  modifies  but 
never  changes  the  general  character  of  the  weather.  The 
intensity  alone  is  ever  altered. 

EECURRENT  TYPES  OF  WEATHER. 

But  of  far  more  importance  is  that  form  of  seasonal 
variation  which  applies  to  the  occurrence  or  recurrence  of 


ANNUAL  AND  SECULAR  VARIATIONS.  313 

similar  weather  about  the  same  date  every  year.  The 
nature  of  recurrent  weather  in  the  temperate  region  of 
variable  pressure  may  be  best  illustrated  by  looking  at 
the  connection  between  the  variable  European  types  and 
the  regular  annual  changes  which  take  place  in  the 
tropics.  In  most  equatorial  and  tropical  climates  there 
are  only  two  or  three  seasons,  which  correspond  to  two  or 
three  positions  of  the  equatorial  low  pressure  and  tropical 
belt  of  anticyclones.  The  monsoons  of  the  Indian  Ocean 
are  the  most  striking  and  best  known  instances  of  weather 
that  recurs  at  the  same  season  of  every  year. 

The  English  word  "  monsoon "  is,  in  fact,  derived 
from  an  Arabic  word  meaning  "season."  But  in  the 
regions  which  lie  between  the  tropical  and  temperate 
zones  the  author  has  found  that  there  are  recurrent 
periods,  intermediate  both  in  their  duration  and  the 
certainty  of  their  return  to  the  monsoons  of  India  and  the 
recurrent  spells  of  European  weather.  One  of  the  best 
known  of  these  is  the  "  Khamsin "  (the  fifty  days),  a 
hot,  sandy  south-east  wind  which  blows  regularly  in 
Egypt  from  the  end  of  March  and  through  April  for 
about  fifty  days.  Klunzinger  has  given  a  whole  series  of 
persistent  and  recurrent  types  for  the  whole  year  in 
Kosseir,  on  the  Ked  Sea,  about  one  hundred  miles  south 
of  Suez  on  the  Egyptian  side.  The  following  may  be 
considered  a  list  of  the  chief  recurrent  periods  of  weather 
in  Great  Britain  and  North-Western  Europe  generally. 

February  7  to  10. — A  spell  of  cold  weather,  associated 
with  the  northerly  type.  This  is  the  first  of  a  series  of 
six  cold  and  three  hot  periods  discovered  by  A.  Buchan. 
He  also  noticed  that  during  the  cold  periods  the  pressure 


31 4  WEATHER. 

was  higher  to  the  north  of  Scotland,  and  lower  to  the 
south,  and  that  during  the  warm  periods  pressure  was 
higher  over  Scotland  than  in  places  to  the  north.  This 
means  that  the  cold  periods  were  the  result  of  the 
occurrence  and  persistence  of  either  the  northerly  or 
easterly  types  of  weather.  We  have  been  unable  to  find 
any  allusion  to  this  spell  in  European  weather  lore. 

March. — The  proverbial  east  winds  of  this  month  are 
mostly  due  to  the  northerly  type  of  weather.  The 
occurrence  of  equinoctial  gales  about  the  21st  of  the 
month  is  almost  universally  believed.  It  is,  however,  a 
curious  fact,  as  has  been  pointed  out  by  K.  H.  Scott,  that 
the  records  of  the  British  observatories  give  no  decided 
indications  of  exceptionally  strong  winds  at  either  equinox. 
Whether  equinoctial  gales  really  occur  in  the  Mediter- 
ranean, and  the  idea  has  been  carried  from  thence  to 
England  by  the  monks,  or  whether  the  weather  in  Great 
Britain  might  not  be  more  properly  called  broken  than 
stormy,  we  cannot  say.  The  author,  however,  rather 
inclines  to  the  latter  view ;  for  it  is  almost  impossible  to 
believe  that  an  idea  which  has  obtained  such  universal 
credence  can  be  altogether  destitute  of  some  real  founda- 
tion. The  difficulties  of  settling  a  question  like  this 
bring  forcibly  before  us  the  uncertainty  of  any  numerical 
estimate  of  climate  or  weather. 

April  11  to  14. — A  cold  spell;  Buchan's  second 
period,  which  he  has  identified  with  the  popular  '•'  weather 
saw  "  of  the  "  borrowing  days." 

May  9  to  14. — A  cold  spell;  Buchan's  third  period. 
This  is  the  most  celebrated  of  the  cold  periods,  as  it 
occurs  over  the  greater  part  of  Europe.  A  good  many 


ANNUAL  AND   SECULAlt   VARIATIONS.  315 

sayings  connected  with  it  are  found  in  many  European 
prognostics,  such  as  those  relating  to  the  frost  saints. 
This  period  is  of  some  interest  on  account  of  the  strange 
theories  which  have  been  propounded  to  explain  the 
origin  of  the  cold.  One  of  the  most  popular  has  been  the 
idea  that  about  the  middle  of  May  the  earth  encountered 
a  stream  of  meteors  which  were  so  numerous  as  to  act 
like  a  cloud  of  dust  and  cut  off  some  portion  of  the  sun's 
heat.  We  need  hardly  say  that  such  an  occurrence  would 
diminish  the  temperature  all  over  the  world,  and  that 
there  is  nothing  to  give  countenance  to  this.  Besides, 
the  passage  of  the  sun's  rays  through  such  a  stone-strewed 
space  could  not  fail  to  give  rise  to  some  kind  o£  blur  of 
light  round  his  disc,  as  when  he  shines  through  big  drops 
of  condensed  vapour.  Nothing  can  be  more  certain  than 
that  this  cold  period  is  usually  due  to  the  setting  in  of  a 
spell  of  the  easterly  or  northerly  type  over  Europe.  At 
any  other  time  of  the  year  the  same  types  bring  similar 
weather. 

June. — A  cold  spell  in  the  second  or  third  week  is 
associated  with  the  northerly  type. 

June  29  to  July  4. — A  cold  spell;  Buchan's  fourth 
period.  Curiously  enough,  we  have  been  unable  to  find  any 
reference  to  these  thermometric  periods  in  the  weather 
lore  either  of  Great  Britain  or  of  any  other  part  of  Europe. 

July  12  to  15. — A  warm  period  ;  Buchan's  first. 

July  15. — St.  Swithin.  The  popular  legend  of  this 
saint,  and  other  rainy  saints  like  St.  Medard,  receives  an 
easy  explanation  from  synoptic  charts. 

August  2  to  8. — A  wet  period ;  the  "  Lammas  floods  " 
of  Scotland. 


316  WEATHER. 

August  6  to  11. — A  cold  period;  Buchan's  fifth. 

August  12  to  15. — A  hot  period ;  Buchan's  second. 
No  prognostics  are  associated  with  these  two  latter 
periods. 

September. — The  easterly  and  northerly  types  are 
rare  during  this  month ;  the  gales  or  broken  weather  at 
the  equinox  are  almost  invariably  of  the  westerly  type. 
About  the  30th  a  fine  period  is  experienced  for  a  few 
days — the  "  Indian  Summer  "  of  North  America. 

October. — About  the  second  or  third  week  a  spell  of 
the  easterly  type  of  moderate  intensity  is  common. 

October  18. — A  fine  quiet  period  about  this  time — 
"St.  Luke's  Summer."  This  and  the  other  summers 
which  occur  at  this  season  have  sometimes  been  stated  to 
be  due  to  the  liberation  of  heat  during  the  condensation 
of  vapour,  and  formation  of  ice,  which  begins  to  take 
place  on  a  large  scale  in  the  polar  regions  soon  after  the 
autumnal  equinox.  According  to  this  theory,  the  opposite 
phenomenon  of  cold  in  April  and  May  is  supposed  to  be 
caused  by  the  absorption  of  heat  due  to  the  melting  of 
ice.  Both  ideas  are  purely  fanciful.  The  spring  cold  we 
have  already  explained ;  the  autumn  summers  are  due  to 
the  recurrence  of  tranquil  periods  at  that  season. 

November  6  to  12. — A  cold  spell;  Buchan's  sixth, 
associated  with  the  northerly  type.  The  llth  is  "  St. 
Martin's  Little  Summer,"  popularly  considered  in  the 
Mediterranean  to  be  a  period  of  warm,  quiet  weather. 

December  3  to  9. — A  warm  period  ;  Buchan's  third. 

The  general  explanation  of  all  these  periodicities  is 
identical.  They  all  depend  for  their  origin  on  a  tendency 
of  certain  types  of  pressure-distribution  to  recur  about 


ANNUAL   AND   SECULAR  VARIATIONS.  317 

the  dates  just  mentioned.  The  cold  periods  all  require 
the  presence  of  the  northerly  or  easterly  types;  the 
warm  periods,  either  of  the  southerly  type  in  winter,  or 
of  anticyclones  in  summer ;  while  wet  or  broken  periods 
indicate  the  recurrence  of  intense  cyclones  of  any  type. 

Eeturning  to  our  old  illustration  of  a  globe  surrounded 
by  a  circulating  atmosphere,  we  can  readily  suppose  that 
at  the  same  date  every  year,  when  the  sun  is  in  the  same 
place,  the  motion  of  the  air  will  tend  to  reproduce  the 
same  kind  of  eddies  in  the  same  localities. 


VALUE  IN  FORECASTING. 

But  now  we  have  to  consider  how  the  knowledge  of 
these  recurrent  periods  can  be  utilized  in  forecasting.  In 
our  last  chapter  on  Diurnal  Variations  we  called  attention 
to  the  nature  of  the  daily  period  of  heat.  In  this,  the 
most  obvious  of  all  meteorological  phenomena,  we  found 
that  though  there  is  a  powerful  heating  influence  present 
every  day,  still  that  other  causes  are  sometimes  so  powerful 
as  to  obliterate  or  invert  the  action  of  the  sun.  As  a 
consequence  of  this  we  cannot  affirm  absolutely  that  the 
night  will  be  colder  than  the  day,  though,  of  course,  such 
is  generally  the  case. 

If  we  were  to  attempt  to  forcast  the  heat  at  any  hour 
by  reference  to  the  mean  curve  of  diurnal  range,  we  should 
sometimes  give  most  erroneous  forecasts ;  if,  on  the  con- 
trary, we  looked  at  the  chart  for  8  a.m.,  we  could  often 
safely  predict  that  the  ensuing  night  would  be  warmer 
than  the  day.  From  the  temperature  diagrams  which 
were  there  given,  we  also  drew  the  important  inference 


318  WEATHEK. 

that,  because  we  do  not  see  a  diurnal  range  every  day,  we 
must  not  infer  that  there  is  no  such  thing  as  a  diurnal 
solar  influence.  If,  then,  such  a  powerful  influence  as  the 
direct  rays  of  the  sun  can  be  so  easily  masked,  we  can 
readily  understand  that  a  weaker  influence,  such  as  the 
declination  of  the  sun  on  any  particular  day,  can  readily 
be  obliterated.  We  can  safely  say  that  the  change  in 
the  altitude  of  the  sun  is  of  secondary  importance,  because 
we  see  every  day  great  changes  in  the  distribution  of 
pressure,  which  are  certainly  in  no  way  related  to  the 
seasonal  change  in  the  declination  of  the  sun.  We  need 
not,  then,  be  surprised  that  the  types  of  heat  or  cold  do 
not  recur  absolutely  every  year,  only  that  there  is  an 
undoubted  tendency  to  do  so.  When  once  we  have 
realized  this,  we  can  easily  understand  the  following 
statement  of  [the  use  of  a  knowledge  of  recurrent  annual 
types  in  forecasting. 

A  forecaster  is  not  justified  in  saying  that  any  period 
will  occur  absolutely ;  still,  when  about  the  time  of  its 
usual  recurrence  the  synoptic  charts  show  signs  of  the 
expected  type,  then  the  forecasts  for  a  few  days  ahead  can 
be  issued  with  greater  confidence.  For  instance,  suppose 
that  about  the  6th  of  November  the  charts  begin  to  show 
traces  of  the  northerly  type,  then — but  not  before — there 
would  be  good  grounds  for  saying  that  a  period  of  cold 
weather,  which  usually  occurs  at  this  season,  has  already 
set  in,  and  may  be  expected  to  last  for  five  or  six  days. 
The  forecaster  is  thus  enabled  to  issue  a  much  longer 
forecast  than  he  can  as  a  rule  safely  attempt. 


ANNUAL  AND   SECULAR  VARIATIONS.  319 


CYCLICAL  PEEIODS. 

By  these  we  mean  periods  which  run  through  their 
whole  course  in  any  time  other  than  a  day  or  a  year. 
Many  investigators  have  thought  that  they  have  detected 
traces  of  periodicities  of  temperature  of  about  twelve  days 
and  of  25*74  days,  the  latter  apparently  connected  with 
the  time  of  the  sun's  rotation.  Others  have  endeavoured 
to  detect  periodicities  of  rain  or  heat  for  longer  epochs, 
especially  one  of  ll'l  years,  which  would  coincide  with  a 
period  of  sun-spots.  As  this  is  the  one  of  most  importance 
and  greater  interest,  and  as  a  discussion  of  it  will  serve  to 
illustrate  the  whole  nature  of  periodicities,  we  shall  con- 
fine our  attention  to  a  short  notice  of  this  cyclic  period 
only. 


SUN-SPOTS  AND  WEATHER. 

Ever  since  the  year  1775,  we  have  more  or  less  com- 
plete records  of  the  relative  extent  of  black  spots  on  the 
sun's  surface.  These  records  show  a  most  unequivocal 
recurrence  of  sun-spot  maxima  at  intervals  of  about 
eleven  years ;  but  the  actual  amount  of  surface  covered 
at  each  maximum  is  very  irregular,  In  the  lower  part  of 
Fig.  63  we  give  a  reduction  of  diagram  which  shows  the 
relative  extent  of  black  spots  on  the  sun  as  plotted  by 
Professor  Balfour  Stewart.  If  we  were  to  draw  over  this 
curve  another  which  showed  the  mean  daily  range  of 
magnetic  declination  for  the  same  year,  we  should  find 
that  there  was  an  unmistakable  similarity  between  the 


320 


WEATHER. 


two  curves,  and  that  both  the  times  and  magnitudes  of 
the  maxima  and  minima  agreed  wonderfully  well. 

In  like  manner  a  curve  which  showed  the  number  of 
auroras  observed  in  each  year  would  also  show  a  striking 
likeness  to  the  curve  of  sun -spotted  area.  This  curve  is 
not  so  valuable  as  that  of  magnetic  declination,  because 
auroras  cannot  be  seen  in  cloudy  weather,  while  magnets 


1-780      1790       1800 


1870 


Inches  -T 

70 


60 


40 


RAINFALL 


ROTHESAY 


L  i  i  it.,  i  n  1 1 1  i  i  i 


1780    1790    l8oO     10      20      30      40      50 

FIG.  63, — Sunspots  and  rainfall. 


60      1070 


can  always  be  observed.  As  these  curves  undoubtedly 
connect  the  state  of  the  sun  with  one  physical  terrestrial 
phenomenon,  and  also  with  another  half-physical,  half- 
meteorological  appearance,  there  would  be  no  inherent 
improbability  in  the  existence  of  a  relation  between  sun- 
spots  and  weather, 

Such  a  relation  would  be  on  quite  a  different  footing 
to  any  quasi-astrological  idea  of  a  connection  between  the 
sun,  moon,  or  stars,  and  weather-changes. 


ANNUAL  AND  SEC'ULAR  VARIATIONS.  321 

Many  investigators  think  that  they  can  trace  some 
kind  ot  connection  between  the  amount  of  rainfall  and 
sun-spots ;  others  see  a  connection  between  the  years  of 
maximum  sun-spots  and  the  frequency  of  cyclones  in  the 
Indian  Ocean  ;  while  some  find  that  marine  casualties 
and  commercial  panics  or  crises  appear  to  follow  a  cycle 
closely  corresponding  to  that  of  sun-spots 

One  great  difficulty  in  deciding  whether  there  is  a 
real  periodicity  in  rain  or  storm  statistics  arises  from  the 
very  irregular  curves  with  which  we  have  to  deal.  All 
the  curves,  on  which  it  is  sought  to  base  the  supposed 
connection  between  sun-spots  and  weather,  have  been  so 
far  smoothed,  that  it  is  difficult  to  say  what  the  result- 
ing curve  really  signifies  and  how  far  true  deductions  can 
be  made  from  it.  This  will  be  better  realized  by  an 
example.  -^On  the  upper  halt*  of  Fig.  63  we  have,  there- 
fore, plotted  the  annual  amount  of  rainfall  at  Rothesay,  in 
Scotland,  for  eighty  years,  over  the  curve  of  sun-spot 
extent  in  the  lower  portion  of  the  diagram.^ Both  curves 
are  purely  the  result  of  observation  and  have  not  been 
smoothed  in  any  way.  ^The  reader  can,  therefore,  draw 
his  own  conclusion  as  to  how  far  there  is  a  real  or  fanciful 
connection  between  the  two  curves.  In  some  points  there 
is  undoubted  similarity  ;  in  others,  an  absolute  contrariety. 
In  the  rainfall  curve,  if  we  take  the  absolute  mathematical 
definition  of  a  maximum — when  any  value  is  greater 
than  either  the  preceding  or  succeeding  ones — there  is 
a  maximum  of  rain  nearly  every  other  year ;  but  if  we 
consider  the  broader  sweeps  of  the  curve,  we  may  find 
more  resemblance.  For  instance,  the  maxima  about  1805, 
1816,  1828,  1837,  1848,  1860,  and  1871  agree  passably 

Y 


322  WEATHER. 

in  both  curves ;  on  the  other  hand,  the  absolutely  greatest 
rainfall  in  the  eighty  years  was  in  1811,  a  year  of  minimum 
spotted  area;  while  another  very  large  rain  maximum 
also  occurred  near  a  time  of  minimum  spots  in  1841. 
Then  again,  the  greatest  minimum  but  one  of  rain,  in 
1870,  occurred  one  year  before  a  maximum  of  spots  and 
only  two  years  before  the  second  largest  maximum  of 
rain.  These  latter  cases,  of  course,  throw  doubt  on 
whether  we  are  justified  in  finding  any  periodicity  at  all 
in  the  rainfall  curve.  Any  attempt  to  smooth  or  alter 
these  curves  by  arithmetical  or  algebraical  processes  can 
only  lead  to  illusive  results ;  we  must  base  our  opinion 
of  the  supposed  connection  between  the  two  curves  on 
our  knowledge  of  other  undoubted  irregular  periodicities. 

In  our  curve  of  thermograms,  Fig.  60,  we  pointed  out 
that  because  there  was  no  obvious  trace  of  diurnal  varia- 
tion of  heat  on  many  days,  we  were  not,  there  fore,  justified 
in  saying  that  there  was  no  such  thing  as  a  diurnal 
heating  influence  of  the  sun.  All  that  could  be  said  was 
that  his  power  had  been  overridden  by  more  powerful 
influences.  In  the  same  way,  the  fact  that  there  are 
heavy  rainfalls  which  have  no  relation  to  the  extent  of 
sun-spotted  area,  does  not  of  itself  prove  that  there  is  no 
connection  between  the  spots  and  weather.  If  we  could 
be  certain  from  any  other  considerations  that  there  was  a 
real  connection  between  the  two  phenomena,  all  that  we 
should  be  justified  in  saying  was,  that  whatever  influences 
the  spots  had  on  weather,  there  were  other  influences 
which  might  be  much  more  powerful. 

Another  great  difficulty  which  we  have  to  face  in 
forming  our  judgment  of  the  possible  connection  between 


ANNITAL  AND  SECULAR  VARIATIONS.  323 

the  state  of  tlie  sun  and  weather,  arises  from  the 
impossibility  of  laying  down  any  absolute  criterion  of 
what  is  a  rainy  year.  Kain  may  be  produced  by  so 
many  different  causes,  and  the  difference  of  amount 
which  is  measured  in  places  near  one  another  is  so  great, 
that  we  are  left  a  great  deal  to  our  own  estimate  of  values 
or  probability.  Thunderstorms  are  the  great  disturbance 
of  rainfall  statistics.  Under  those  circumstances  as  much 
as  two  inches  of  rain  may  fall  in  one  place,  and  but  a  few 
drops  in  another  only  a  few  miles  distant.  Yearly  totals 
show  the  same  discrepancies. 

For  example,  in  the  year  1872 — a  year  of  sun-spot 
maximum — Buchan  has  plotted  the  rainfall  within  the 
limited  area  of  Scotland.  He  finds  that  while  near 
Aberdeen  the  rainfall  was  seventy-five  per  cent,  above 
the  average,  the  amount  at  Gape  Wrath,  about  one 
hundred  miles  distant,  was  below  the  average.  Then,  of 
course,  the  difficulty  is,  why  should  we  take  the  returns 
of  one  station  more  than  another  to  compare  with  sun- 
spots,  when  the  latter  affect  the  whole  world  simul- 
taneously ? 

Kain  in  the  abstract  is  a  mere  entity — we  must  say 
what  kind  of  rain  it  was  which  fell.  Was  it  cyclone  rain, 
or  secondary  rain,  or  that  associated  with  thunderstorms  ? 
The  true  criterion  of  periodicity  requires  not  only  an 
amount  of  rain  which  corresponds  with  the  state  of  the 
sun's  surface,  but  also  rainfall  under  the  same  conditions 
of  atmospheric  pressure.  We  must  not  compare  the  rain 
of  cyclones  with  the  rain  of  thunderstorms,  unless  we  can 
show  that  they  may  both  be  produced  by  increased 
intensity  of  the  weather  generally.  This  is,  however, 
sometimes  the  case. 


324  WEATHER. 

There  is  another  point  which  must  be  remembered  in 
the  discussion  of  questions  as  to  the  connection  between 
the  sun  and  the  weather.  We  have  shown  that  weather 
in  the  temperate  zone  is  the  product  of  the  passage  of 
cyclones,  anticyclones,  etc.,  so  that  we  cannot  properly 
talk  of  the  influence  of  any  physical  cause  on  weather  in 
the  abstract.  We  must  thiuk  how  the  physical  cause 
would  act  on  the  general  circulation  of  the  atmosphere. 
When  we  discussed  the  daily  influence  of  the  sun  on 
weather,  we  showed  how  heat  modified  in  a  different 
manner  the  ascending  currents  of  a  cyclone  or  the 
descending  ones  of  an  anticyclone. 

In  the  same  way,  if  the  condition  of  the  sun's  surface 
does  affect  weather,  the  action  must  take  place  through 
the  medium  of  cyclones  and  anticyclones.  We  must,  in 
fact,  show  that  in  years  of  sun-spot  maxima  and  minima, 
the  circulation  of  the  atmosphere  is  either  more  intense 
generally,  or  that  the  formation  of  cyclones,  etc.,  is  then 
in  some  manner  modified. 

This  view  of  the  true  nature  of  solar  action  explains 
some  anomalies  which  the  advocates  of  the  sun-spot 
theory  have  been  unable  to  explain.  They  find  that  in 
j^ome  places  the  maxima  of  spots  are  associated  with  the 
minima  of  rain.  If  we  try  to  connect  rainfall  and  sun- 
spots  in  the  abstract,  we  are  helpless  to  explain  the 
discrepancy.  But  if,  on  the  contrary,  we  realize  that 
an  alteration  in  the  solar  heat  may  modify  the  formation 
of  cyclones,  then  we  can  at  once  explain  the  apparent 
contradiction  of  results.  For  instance,  in  the  year  1872, 
to  which  we  have  already  alluded,  the  general  position  of 
cyclone  centres  over  North- Western  Europe  was  con- 


ANNUAL  AND  SECULAR  VARIATIONS.  325 

siderably  displaced.  Instead  of  lying  to  the  west  of 
Scotland,  the  centre  of  cyclone  activity  appeared  to  lie 
between  England  and  Norway.  This,  of  course,  made 
England  wetter,  and  the  north-west  of  Scotland  drier 
than  usual ;  but  it  will  take  many  years  before  we  are 
justified  in  saying  that  this  displacement  was  due  to  the 
influence  of  solar  spots. 

It  is,  no  doubt,  a  very  tempting  ideal  to  look  at  the 
sun  as  the  prime  mover  of  the  atmosphere,  and  to  en- 
deavour to  follow  variations  in  the  heat  or  energy  of  his 
action  into  their  final  products  as  wind  or  rain.  But 
when  we  consider  what  the  real  nature  of  weather  is,  as 
revealed  to  us  by  means  of  synoptic  charts,  we  see  at 
once  that,  though  undoubtedly  an  alteration  in  the  sun's 
power  would  sooner  or  later  be  reflected  in  his  results, 
any  attempt  to  deduce  one  from  the  other  directly  must 
lead  to  disastrous  failure. 


RELATION  TO  FORECASTING. 

Though  opinions  will  doubtless  differ  as  to  whether 
we  are  justified  in  asserting  that  there  is  any  connec- 
tion between  sun-spots  and  weather,  there  is  no  uncer- 
tainty as  to  what  the  value  of  that  knowledge  would  be 
to  a  forecaster. 

The  author  believes  himself  that  there  are  signs  of 
some  real  relation  between  the  extent  of  spots  on  the 
sun's  surface  and  the  rainfall  curve  at  Bothesay,  but 
how  should  we  fare  if  we  tried  to  forecast  the  rainfall 
for  any  particular  year?  The  most  cursory  glance  at 
the  two  curves  of  sun-spots  and  rainfall  will  show  that, 


326  WEATHER. 

if  we  were  to  attempt  to  forecast  rainfall  on  the  assump- 
tion that  the  amount  would  follow  the  sun-spot  curve,  we 
should  get  just  the  same  unsatisfactory  results  as  if  we 
attempted  to  forecast  the  temperature  at  different  hours 
by  reference  to  the  mean  diurnal  curve  of  heat.  Every 
meteorological  element  depends  for  its  value  on  the 
balance  of  several  nearly  equal  forces,  so  that  an  attempt 
to  forecast  the  resulting  value  by  means  of  the  variations 
of  one  of  these  forces  can  only  lead  to  failure. 

So  far  for  the  use  of  the  knowledge  even  of  a  certain 
cyclic  period  in  forecasting  the  character  of  a  year  as  a 
whole  ;  and  it  is  still  more  impossible  to  use  any  abstract 
periodicity  in  forecasting  the  weather  for  any  particular 
day.  We  shall  see  in  a  future  chapter  that  all  weather 
prevision  depends  on  the  estimate  which  an  experienced 
forecaster  can  make  as  to  the  probable  path  of  any 
cyclone,  or  as  to  the  formation  of  a  new  one.  How  much 
would  the  abstract  knowledge  that  it  was  a  maximum  or 
minimum  sun-spot  year  help  him  to  form  such  a  judg- 
ment ?  Obviously  nothing. 

On  the  whole,  then,  we  may  say  that  though  there 
are  certainly  very  strong  grounds  for  the  belief  that  there 
is  some  real  connection  between  the  state  of  the  sun's 
surface  and  terrestrial  weather,  still,  from  the  nature  of 
atmospheric  circulation,  we  are  unable  to  utilize  this 
fact  in  forecasting  weather,  either  for  any  season  or  for 
any  day. 


(     327 


CHAPTER  XIIL 
TYPES  AND  SPELLS  OF  WEATHER. 
INTRODUCTORY. 

IN  the  foregoing  chapters  we  have  devoted  our  atten- 
tion more  to  the  nature  of  the  causes  which  produce 
weather  at  any  moment  than  to  the  sequence  of  weather 
for  several  consecutive  days.  We  have,  in  fact,  rather 
described  the  nature  of  the  individual  disturbances  which 
form,  as  it  were,  the  units  of  weather,  than  the  manner 
in  which  these  components  move  or  follow  one  another. 
The  word  "weather"  is  used  by  meteorologists  in  a  twofold 
sense.  When  they  talk  of  the  weather  at  a  moment, 
they  use  the  word  in  a  restricted  signification,  referring  to 
the  appearance  of  the  sky,  or  to  the  occurrence  of  rain, 
snow,  etc.  When  they  talk  of  weather  for  a  longer 
period,  as,  for  instance,  a  wet  week,  or  a  cold  month, 
they  use  the  word  in  a  more  extended  sense,  and  include 
the  sequence  of  every  meteorological  element  for  the 
time  in  question. 

We  have  already  mentioned  that  in  the  temperate 
zone  the  units  of  weather,  such  as  cyclones  or  anticyclones, 
are  perpetually  moving  or  altering  their  shape,  and 
thereby  producing  changes  of  weather ;  or  to  put  it 


328  WEATHER. 

more  formally,  weather  in  the  temperate  zone  is  the  pro- 
duct of  the  passage  of  cyclones,  anticyclones,  or  of  the 
minor  forms  of  isobars. 

We  have  also  pointed  out  that  all  forecasting  depends 
on  the  limited  power  which  we  possess  of  knowing  before- 
hand what  the  path  of  any  disturbance  is  likely  to  be, 
or  what  new  changes  in  the  distribution  of  pressure  will 
probably  take  place.  For  instance,  if  we  see  a  cyclone 
approaching  our  own  country  at  eight  o'clock  in  the 
morning,  how  can  we  tell  in  what  direction  it  will  move, 
or  if  it  is  likely  to  grow  more  or  less  intense  ?  If  we 
see  an  anticyclone,  are  there  any  signs  by  which  we  can 
know  whether  it  is  going  to  remain  stationary,  or  to 
break  up  and  disappear  ? 

When  we  have  examined  a  very  large  number  of 
synoptic  charts  we  soon  see  that,  though  no  two  are  alike, 
there  is  much  in  common  so  far  as  their  sequence  is  con- 
cerned. Though  a  cyclone  may  move  in  any  direction, 
and  almost  with  any  velocity,  nothing  is  a  matter  of 
accident,  but  certain  types  of  motion  are  associated  with 
certain  types  of  general  pressure  distribution. 

Our  purpose  in  this  chapter  is  to  explain  the  nature 
of  these  changes,  by  giving  in  some  detail  the  four  great 
types  of  weather  which  occur  in  Western  Europe,  with 
shorter  notices  of  those  in  the  United  States  and  in  the 
tropics.  In  doing  so  we  will  bear  in  mind  the  twofold 
object  of  all  scientific  meteorology — the  explanation  of 
past  weather  by  reference  to  the  motion  of  cyclones,  etc., 
and  the  classification  of  typical  changes  with  reference  to 
future  forecasts. 

Long  verbal   descriptions     of  complicated   weather- 


TYPES  AND  SPELLS   OF   WEATHER.  329 

maps  are  not  only  tedious,  but  unintelligible  to  all 
except  those  who  have  made  synoptic  charts  their  special 
study.  As  our  object  is  to  convey  an  idea  of  the  nature 
of  weather-changes  to  those  who  have  no  previous  know- 
ledge of  the  subject,  we  shall,  therefore,  rather  trust  to 
copious  illustrations  of  carefully  selected  specimens,  and 
the  reader  must  look  at  them  and  supply  his  own 
descriptions.  By  this  means  he  will  learn  the  character 
of  atmospheric  changes  and  the  ways  of  cyclones  by  eye, 
rather  than  by  reference  to  any  written  formula.  He 
will  see  the  rapidity  with  which  these  changes  take 
place,  and  acquire  that  knowledge  of  the  nature  of 
weather  which  will  enable  him  to  form  a  just  conception 
of  the  great  problems  of  forecasting. 

We  shall  assume  that  he  has  so  far  mastered  the 
preceding  chapters  that,  when  we  talk  of  a  cyclone,  he 
knows  that  it  is  equivalent  ta  bad  weather — warmth  in 
front,  cold  in  rear,  wind  according  to  intensity  ;  and  that 
when  we  say  an  anticyclone  covers  any  country,  that 
means  generally  fine  weather — always  light  wind,  but 
blue  sky,  mist,  heat,  or  cold,  according  to  the  circum- 
stances of  latitude  or  season.  Also  that  the  direction 
of  wind  is  given  at  once  by  naming  an  isobaric  shape  or 
any  portion  of  it. 

Our  illustrative  charts,  mostly  on  a  uniform  scale 
and  projection,  embrace  an  area  that  extends  from  the 
Kocky  Mountains  to  Moscow,  and  from  the  equator  to 
Greenland.  In  all,  pressures  of  29*9  ins.  (760  mm.),  and 
all  above,  are  marked  by  full  isobars,  while  those  below 
are  dotted,  so  that  the  reader  sees  at  a  glance  the  broad 
elative  distribution  of  high  and  low  pressure* 


330  WEATHER. 

DISTRIBUTION  OF  PRESSURE  OVER  THE  GLOBE. 

Over  the  above  area  the  distribution  of  atmospheric 
pressure  presents  certain  constant  features,  namely — 

1.  An  equatorial  belt  of  nearly  uniform  low  pressure. 

2.  A  tropical  belt  of  high  pressure  rising  at  intervals 
into  great  irregular  elevations  or  anticyclones. 

3.  A  temperate  and  arctic   region  of  generally  low 
pressure,  but  in  which  occasionally  areas  of  high  pressure 
appear  for  a  considerable  period. 


WEATHER  IN  THE  DOLDRUMS. 

The  equatorial  belt  constantly  covers  the  Sahara  and 
the  Amazon  valley,  and  always  narrows  over  the  Atlantic 
at  about  30°  west  longitude,  where  it  often  does  not 
reach  higher  than  10°  north  latitude.  The  shape  and 
depth  of  this  area  are  tolerably  constant. 

This  is  the  "doldrums"  of  the  Atlantic  navigators. 
Our  charts  only  show  the  north  side  of  this  area;  the 
south  side  is  formed  by  an  anticyclone,  which  always 
lies  over  the  South  Atlantic.  The  doldrums,  therefore, 
form  a  sort  of  long  hollow,  or  col,  between  two  anti- 
cyclones, and  though  on  the  one  side  the  north-east  trade 
blows  moderately,  and  the  south-east  trade  on  the  other, 
still  in  the  centre  there  must  be  calm.  This  is  well 
shown  in  Fig.  68,  where  we  see  the  symbol  of  calm 
between  the  two  trades.  These  sultry  doldrums  are 
much  dreaded  by  sailors,  for  in  them  "a  ship  may  lie 
for  weeks  on  the  hot  smooth  water,  under  a  cloudless 
sky,  with  pitch  oozing  from  her  decks ;  a  region  ol  un- 


TYPES  AND  SPELLS  OF  WEATHEE.  331 

bearable  calm,  broken  occasionally  by  violent  squalls, 
torrential  rain,  and  fearful  lightning  and  thunder."  The 
general  appearance  of  the  sky  is  a  steamy  haze,  some- 
times growing  into  a  uniform  gloom,  with  or  without 
heavy  rain;  at  other  times  gathering  into  small  ill- 
defined  patches  of  soft  cumulus,  or  the  forms  of  cloud 
given  in  Fig.  13,  a,  b,  and  c.  After  dark  there  is  always 
a  great  development  of  sheet  lightning  till  about  two  in 
the  morning.  As  the  position  of  this  area  only  varies 
very  slowly  in  its  annual  course  a  little  more  north  or  a 
little  more  south,  there  is  nothing  to  change  the  weather, 
which  therefore  remains  of  a  uniform  character. 


WEATHER  IN  THE  TRADE-WINDS. 

The  tropical  belt  comprises  a  region  of  high  pressure, 
rising  at  variable  intervals  into  great  anticyclones.  These 
anticyclones  are  usually  the  longest  in  an  east  and  west 
direction,  and  often  rise  into  two  or  more  heads.  Their 
position  is  generally  variable,  with  the  exception  of  one, 
which  is  always  found  over  the  central  Atlantic.  This 
anticyclone  forms  a  very  important  factor  of  the  weather 
both  of  Western  Europe  and  of  the  United  States,  and 
will  be  constantly  referred  to  as  "the  Atlantic  anticy- 
clone/' The  extension  south  and  west  of  this  anticyclone 
is  tolerably  constant,  while  north  and  east  it  is  variable, 
sometimes  rising  as  far  as  60°  north,  and  stretching  over 
Great  Britain  and  Continental  Europe. 

The  wind  blows  round  this  as  in  all  anticyclones. 
The  north-east  and  east  winds  on  the  southern  side  of  the 
Atlantic  anticyclone  constitute  the  celebrated  "trade- 


332  WEATHER. 

winds."  An  inspection  of  Figs.  68-71  will  both  show 
their  true  nature  and  correct  some  popular  fallacies  as 
to  their  position  and  constancy.  It  is  obvious,  from  the 
nature  of  anticyclone  winds,  that  north  or  north-east 
winds  must  stretch  far  north  on  the  easterly  edge,  which 
accounts  for  the  north-east  trade  being  often  picked  up 
off  the  coast  of  Portugal.  But  on  the  westerly  edge  of 
the  anticyclone  the  wind  must  be  more  south-east  or 
south,  and  in  practice  is  lighter  and  more  variable. 
The  centre  must  be,  and  is,  calm,  so  that  the  wind-maps 
which  appear  in  physical  atlases  with  the  north-east 
trade  described  as  a  belt  of  wind  parallel  to  the  equator 
are  most  delusive.  The  degree  of  constancy  in  the  direc- 
tion and  force  of  the  trades  is  best  gathered  from  an 
inspection  of  the  charts.  We  then  see  at  once  that  the 
position  of  the  edges  of  the  anticyclone  is  perpetually 
changing,  and  that  the  gradients  are  very  variable;  so 
that,  as  a  matter  of  course,  both  the  direction  and 
strength  of  the  trades  vary  very  considerably. 

The  weather  in  the  trades  is  usually  fine,  and  the 
sky  more  or  less  covered  with  a  peculiar  small  detached 
cumulus,  often  called  "  trade  cumulus."  This  is  a  small 
isolated  cloud  bending  backwards  from  the  flat  base,  as 
in  Fig.  11,  a,  in  our  chapter  on  Clouds,  which  often 
degenerates  into  the  small  lens-shaped  mass  as  in  Fig. 
13,  e.  Sometimes  a  thin,  hard,  broken  strato-cumulus 
covers  the  sky  with  such  regularity  that,  when  seen  in 
perspective  near  the  horizon,  we  look  at  a  series  of  bars, 
like  the  leaves  of  a  Venetian  blind;  but  if  the  gradients 
are  all  steep,  squalls  and  showers  from  cumulus  cloud 
are  of  frequent  occurrence  in  the  trade -wind  regions. 


TYPES  AND   SPELLS  OF  WEATHER.  333 

Cyclones  are  rarely,  if  ever,  formed  to  the  south  of 
this  Atlantic  anticyclone ;  sometimes,  however,  they  have 
their  origin  on  its  south-west  side,  when  they  work 
round  the  anticyclone,  first  towards  the  north-west,  and 
then  towards  north-east.  These  are  the  West  India 
hurricanes. 

The  north  side  of  the  anticyclone  is  the  birthplace 
of  innumerable  cyclones  of  every  size  and  intensity,  which 
invariably  move  towards  some  point  of  east.  These  are 
the  cyclone-storms  which  affect  Europe. 

Cyclones  are  also  occasionally  formed  on  the  south- 
east side  near  Madeira ;  these  either  work  very  slowly 
round  the  high  pressure  to  the  south-west,  or  else  leave 
the  anticyclone  and  go  east  over  the  Straits  of  Gibraltar. 
In  winter-time,  another  anticyclone  generally  lies  over 
Mexico,  and  the  col  between  this  and  the  Atlantic  anti- 
cyclone forms  the  most  prominent  feature  in  the  meteor- 
ology of  the  United  States  sea-board. 

WEATHER  IN  TEMPERATE  ZONE. 

The  temperate  and  arctic  region  extends  from  the 
tropical  high  pressure  to  the  pole.  Though  ordinarily 
low,  the  pressure  is  perpetually  fluctuating  by  reason  of 
the  incessant  passage  of  cyclones ;  yet  occasionally  per- 
sistent areas  of  high  pressure  appear  in  certain  portions 
of  it. 

As  a  necessary  consequence  of  this,  the  weather  in 
this  zone  must  be  changeable,  with  variable  winds. 

From  this  brief  survey,  we  see  at  once  the  broad 
features  of  the  climates  of  the  world — the  persistent 


334}  WEATHER. 

equatorial  calms  and  rains,  the  regular  trades  of  the 
tropics,  and  the  variable  wind  and  weather  of  the  tempe- 
rate zone. 

We  will  now  proceed  to  examine  the  weather  of  the 
temperate  zone  in  some  detail. 

TYPES  OF  PRESSURE  IN  TEMPERATE  ZONE. 

In  spite  of  the  great  variability  of  the  temperate 
zone,  there  are — with  reference  to  Western  Europe — at 
least  four  constant  types  of  weather  which  coincide 
with  four  distinct  types  of  pressure-distribution. 

1.  The  southerly,  in  which  an  anticyclone  lies  to  the 
east  or  south-east  of  Great  Britain,  while  cyclones  coming 
in  from  the  Atlantic  either  beat  up  against  it  or  pass 
towards  north-east. 

2.  The  westerly,  in  which  the  tropical  belt  of  anti- 
cyclones is  found  to  the  south  of  Great  Britain,  and  the 
cyclones  which  are  formed  in  the  central  Atlantic  pass 
towards  east  or  north-east. 

3.  The  northerly,  in  which  the  Atlantic  anticyclone 
stretches  far  to  the  west  and  north-west  of  Great  Britain, 
roughly  covering   the   Atlantic   Ocean.      In   this   case, 
cyclones  spring  up  on  the  north  or  east  side,  and  either 
work  round  the  anticyclone  to  the  south-east,  or  leave 
it  and  travel  rapidly  towards  the  east. 

4.  The  easterly,  in  which  an  apparently  non-tropical 
anticyclone  (or  one  disconnected  with  the  tropical  high- 
pressure  belt)  appears  in  the  north-east  of  Europe,  rarely 
extending  beyond  the  coast-line,  while  the  Atlantic  anti- 
cyclone is  occasionally  totally  absent  from  the  Bay  of 


TYPES   AND   SPELLS   OF   WEATHER.  335 

Biscay.  The  cyclones  then  either  come  in  from  the 
Atlantic  and  pass  south-east  between  the  Scandinavian 
and  Atlantic  anticyclones,  or  else,  their  progress  being 
impeded,  they  are  arrested  or  deflected  by  the  anticyclone 
in  the  north-east  of  Europe.  Sometimes  they  are  formed 
to  the  south  of  the  Scandinavian  anticyclone,  and  advance 
slowly  towards  the  east,  or  sometimes  even  towards  the 
west. 

These  types  are  so  named  because  the  prevailing  wind 
in  each  is  from  south,  west,  north,  and  east  respectively. 
The  connection  of  these  European  groups  with  those  of 
the  United  States  will  be  considered  under  the  details  of 
each  type. 

Notice  will  now  be  directed  to  the  details  of  these 
types — first  to  their  main  character  and  seasonal  modifi- 
cations, together  with  the  indications  of  intensity,  and 
then  to  any  signs  of  persistence  or  change  of  type  when 
possible. 

But  however  much  we  study  details,  the  above  general 
view  of  the  distribution  of  pressure  in  the  earth's  surface 
must  never  be  forgotten,  as  without  that  we  lose  the  only 
clue  to  the  ceaseless  and  complicated  changes  with  which 
we  have  to  deal. 

SOUTHERLY  TYPE. 

In  this  type  the  Atlantic  anticyclone  extends  very 
little  to  the  northward ;  another  of  the  tropical  belt  of 
anticyclones  covers  Mexico  or  the  southern  states  of  the 
American  Union ;  while  a  third  area  of  high  pressure 
covers  Northern  and  Eastern  Europe. 


336  WEATHER. 

The  North  Atlantic  is  occupied  by  a  persistent  area 
of  low  pressure  in  which  cyclones  are  constantly  being 
formed  ;  these  beat  up  against  the  high  European  pressure, 
and  either  die  out  or  are  repelled. 

Sometimes,  especially  in  summer,  small  cyclones 
arising  in  the  easterly  side  of  the  area  of  depression  pa*s 
rapi'ily  near  the  British  coasts  in  a  north  or  north-east 
direction.  In  either  case  it  is  somewhat  rare  for  the 
centre  of  a  cyclone  to  reach  the  coast-line  of  Europe,  so 
that  generally  Great  Britain  is  under  the  influence  of 
the  rim  or  edge  of  either  a  cyclone  or  anticyclone. 

At  other  times  the  Atlantic  low  pressure  extends  over 
Great  Britain,  driving  the  high  pressure  eastwards,  with- 
out forming  any  definite  cyclone.  In  this  ease,  the  indi- 
cations are  for  tolerably  fine  weather  and  little  wind, 
with  a  very  low  barometer — a  condition  which  often 
excites  remark. 

This  type  of  weather  occurs  at  all  seasons  of  the  year, 
but  it  is  most  common  and  persistent  in  winter ;  in  fact, 
the  warmth  or  otherwise-  of  the  winter  principally  depends 
on  the  number  of  days  of  this  type. 

No  definite  sequence  of  weather  to  the  United  States 
is  connected  with  the  occurrence  of  this  type  in  Europe. 
While  the  Mexican  anticyclone  is  tolerably  persistent, 
cyclones  which  form  in  the  Hudson's  Bay  Territory 
usually  pass  into  the  Atlantic  and  are  lost  there ;  but  at 
the  same  time  another  totally  different  class  of  cyclone 
forms  in  the  col  which  lies  between  the  Atlantic  and 
Mexican  anticyclones,  and  moves  along  the  northern  edge 
of  the  former  till  they  reach  Europe.  The  centres,  of 
course,  never  touch  the  American  continent,  but  the 


TYPES  AND  SPELLS  OF  WEATHER. 


337 


gales  associated  with  the  western  side  of  these  cyclones 
often  do  much  damage  to  the  United  States  coast. 

The  above  will  be  more  easily  understood  by  reference 
to  an  actual  example.  In  Figs.  64-67,  we  give  charts  over 
a  large  area,  for  the  four  days  November  10-13,  1877,  at 
7.35  a.m.  Washington.  None  of  these  show  the  ZODC  of 


FIG.  64. — Southerly  type  of  weather. 

equatorial  low  pressure,  but  in  all  the  tropical  belt  of 
anticyclones  and  the  temperate  and  Arctic  zone  of  low 
pressure  are  very  obvious.  In  all  we  find  three  persistent 
anticyclones,  one  over  the  lower  Mississippi  valley,  another 
in  mid- Atlantic,  and  a  third  over  Moscow.  The  North 
Atlantic  and  Hudson's  Bay  Territory  are  covered  by  low 


338 


WEATHER. 


pressure,  and  this  area  is  the  theatre  of  the  formation  of 
an  incessant  series  of  new  cyclones,  whose  history  we  are 
now  going  to  trace. 

But  first  let  us  consider  the  southern  edges  of  the 
tropical  anticyclones.  The  east  winds  under  the  American 
high  pressure  are  the  trade-winds  of  Cuba  and  the  Central 


FIG.  65. — Southerly  type  of  weather. 

American  republics  as  shown  by  the  small  arrows ;  the 
Atlantic  anticyclone  gives  the  regular  trades  of  that 
ocean,  and  the  anticyclone,  whose  edge  we  see  in  the 
chart  over  Moscow,  really  extends  over  the  whole  of 
Siberia,  and  gives  the  north -east  monsoon  to  the  Indian 
Ocean.  This  all  shows  in  a  very  striking  manner  the 


TYPES  AND   SPELLS   OF  WEATHER. 


339 


dependence  of  weather  in  different  parts  of  the  world  on 
each  other,  and  also  the  true  nature  of  the  problems 
which  the  meteorologist  has  to  solve.  The  cyclone  which 
covered  Great  Britain  on  November  10, 1877,  had  its  origin 
in  the  Atlantic  anticyclone  which  dominates  the  trade- 
winds.  Its  eastward  path  was  deflected  by  the  Asiatic 


ioo  So 


FIG.  G6. — Southerly  type  of  weather. 

anticyclone  which  caused  the  north-east  monsoon  in 
Calcutta,  and  its  intensity  was  increased  by  a  depression 
which  passed  into  the  Atlantic  from  the  Hudson's  Bay. 
At  the  same  time  the  actual  force  of  the  wind  was 
determined  at  every  station  by  the  exposure  ;  every  hill 
drew  a  little  more  or  less  rain,  every  tidal  river  brought 


340 


WEATHER. 


up  local  showers.  It  is  this  combination  of  the  very 
large  with  the  very  small  which  constitutes  one  of  the 
great  difficulties  of  meteorology,  and  all  the  skill  of  the 
meteorologist  is  required  to  assign  to  each  influence  its 
proper  place  and  value.  He  cannot  explain  the  weather 
on  any  day  without  casting  his  eyes  over  the  whole 


FIG.  67. — Southerly  type  of  weather. 

northern  hemisphere  and  round  the  little  hills  and  valleys 
which  bound  his  own  horizon. 

Keturning  now  to  our  cyclones  north  of  the  tropical 
belt  of  anticyclones.  On  November  10  a  well-defined 
cyclone  lay  between  Scotland  and  Iceland,  a  V-depression 
lay  in  the  col  between  the  Atlantic  and  American  anti- 


TYPES   AND   SPELLS   OF  WEATHER.  341 

cycloDes,  while  another  cyclone  covered  the  Hudson's 
Bay.  Arrows  show  the  general  direction  of  the  wind  in 
the  leading  capitals  and  cities,  and  partially  the  varying 
velocity. 

By  next  day,  the  llth,  the  position  and  shape  of  the 
European  cyclone  had  scarcely  changed,  but  the  depth 
had  increased  no  less  than  six-tenths  of  an  inch  (15  mm.), 
while  the  position  of  the  isobar  of  30'0  inch  remained 
the  same  over  Europe.  The  Atlantic  V  and  the  Hudson's 
Bay  cyclone  have  disappeared  and  apparently  been 
.merged  into  the  great  depression  which  now  fills  the 
whole  North  Atlantic.  The  nature  of  this  change  should 
be  carefully  considered,  as  it  is  most  typical  of  Atlantic 
weather,  and  shows  the  nature  of  what  the  meteorologist 
has  got  to  deal  with,  and  the  impossibility  of  ever 
arriving  at  any  calculation  as  to  cyclone  paths.  If  a 
cyclone  would  only  keep  a  tolerably  regular  shape,  and 
move  in  even  a  moderately  definite  path,  weather  fore- 
casting would  be  one  of  the  most  certain  and  definite  of 
sciences.  But  when,  as  here,  two  or  three  cyclones 
gather  themselves  up  into  a  new  formation  within  twenty- 
four  hours,  there  is  nothing  definite  to  trace.  We  cannot 
say  how  the  Hudson's  Bay  cyclone  has  moved  into  the 
Atlantic,  even  if  it  is  correct  to  say  that  it  had  done  so 
at  all.  However,  such  is  the  way  of  cyclones,  and  our 
object  here  is  to  explain  it  all  as  best  we  can.  We  often 
see  a  precisely  analogous  action  when  watching  the  flow 
of  a  river.  The  impulse  of  two  or  three  small  eddies 
seems  to  form  one  big  one  in  a  new  place. 

The  effect  of  these  changes  on  Western  Europe  would 
be  to  cause  a  rapid  fall  of  the  barometer — from  surge, 


342  WEATHER. 

not  from  advance  of  a  cyclone — and  to  increase  the 
steepness  of  the  gradients  with  the  general  intensity  of 
the  weather. 

The  irregular  bends  in  the  isobar  of  30'0  ins.  (763 
mm.)  over  Europe  should  be  noted,  for  they  are  due  to 
small  secondaries,  and  indicate  rain  without  wind  in  their 
respective  districts. 

Below  the  cyclone-region,  the  Atlantic  and  American 
anticyclones  are  joined  by  an  arm  of  high  pressure,  while 
a  very  pronounced  depression  appears  over  the  Bermudas. 

On  the  following  day,  November  12  (Fig.  66),  though 
the  position  of  the  centre  and  depth  of  the  European 
cyclone  are  still  unchanged,  the  area  of  low  pressure  has 
extended  over  the  whole  of  Europe,  which  is  now  covered 
by  a  mass  of  secondaries ;  and  the  isobar  of  3OO  ins. 
(763  mm.)  has  been  pushed  a  little  eastwards.  Observe 
that  the  line  of  weakness,  across  which  the  cyclone  en- 
deavours .  to  pass,  is  the  col  between  the  Atlantic  and 
Siberian  anticyclones. 

The  loop  in  the  isobars  which  lay  over  Bermuda  on 
the  previous  day  has  now  moved  to  the  north-east  and 
developed  into  a  moderate  cyclone,  while  a  third  de- 
pression appears  over  Hudson's  Bay. 

Now  look  at  the  last  chart  (Fig.  67)  for  November 
13,  and  try  to  say  how  it  is  related  to  the  previous  figure. 
The  European  cyclone  is  now  represented  by  an  irregular 
depression  over  Iceland,  whose  lowest  point  is  06  in. 
(15  mm.)  above  the  level  of  the  previous  day,  but  the 
general  sweep  of  the  isobars  unquestionably  connect  this 
with  another  depression  in  mid-Atlantic.  The  latter 
certainly  represents  the  cyclone  which  lay  over  that 


TYPES  AND   SPELLS   OF  WEATHER.  343 

region  in  the  preceding  chart,  much  diminished  in  in- 
tensity, and  partially  coalesced  with  the  Hudson's  Bay 
depression.  The  European  secondaries  of  the  previous 
day  are  now  represented  by  a  well-marked  deflection  of 
the  isobars  over  the  Gulf  of  Lyons.  We  can  describe  all 
this,  but  how  can  we  trace  the  history  of  each  individual 
depression  ? 

While  the  weather  in  the  Atlantic  has  diminished  in 
intensity,  the  low  pressure  over  Southern  Europe  has 
extended  into  Africa ;  but  in  spite  of  all  these  changes, 
the  position  of  the  isobar  of  3OO  ins.  (763  mm.)  remains 
very  stationary  over  the  eastern  shores  of  the  Baltic. 

Now,  though  different  totally  in  detail,  these  changes 
are  exactly  analogous  to  the  fusion  of  various  cyclones 
into  new  configurations  which  occurred  in  the  previous 
days,  and  similar  changes  would  continue  as  long  as  this 
type  of  weather  lasted.  We  might  describe  the  whole 
roughly  by  saying  that,  while  the  anticyclones  remained 
stationary,  the  generally  low  area  of  the  North  Atlantic 
was  the  theatre  of  the  incessant  formation  and  breaking 
up  of  cyclones. 

We  do  not  purpose  going  into  the  details  of  the 
weather-sequence  during  this  type  in  any  one  place  or 
country,  but  the  broad  features  in  Western  Europe  to  a 
solitary  observer  are  very  simple. 

As  atmospheric  pressure  falls,  temperature  rises,  and 
the  sky  grows  dirtier  till  drizzling  rain  sets  in.  The 
wind,  from  some  point  of  south,  having  backed  slightly, 
rises  in  velocity  till  the  barometer  has  reached  its  lowest 
point.  As  soon  as  pressure  begins  to  increase,  the  wind 
veers  a  little  and  gradually  falls,  the  air  becomes  cooler, 


314  WEATHER. 

and  the  sky  begins  to  clear ;  but  the  clouds  rarely  become 
hard,  or  form  well-defined  cumulus.  By  next  day,  per- 
haps, the  same  sequence  is  repeated,  varying  only  in 
intensity,  but  not  in  general  character,  and  this  alteration 
perpetually  lasts  for  weeks  at  a  time. 

The  temperature  of  this  type  is  always  high,  partly 
because  of  the  prevailing  southerly  winds,  and,  as  the 
cyclones  die  out,  the  slight  degree  of  cold  which  follows 
is  very  noticeable.  Sometimes  a  portion  of  the  Russian 
anticyclone  reaches  Great  Britain,  and  in  winter  white 
frost  of  short  duration  would  ensue.  The  air  is  always 
damp,  principally  from  the  action  of  southerly  winds,  and 
for  the  same  reason  the  sky  is  usually  dull  or  overcast. 
The  wind  is  remarkable  for  its  steadiness,  both  in  direction 
and  the  way  of  blowing.  This  results  from  the  large 
scale  on  which  the  cyclonic  action  takes  place. 

So  far  for  the  explanation  of  weather  after  it  has 
passed,  but  we  may  now  consider  how  this  example  illus- 
trates the  nature  of  forecasting.  Beginning  in  the  west, 
the  United  States  forecaster  has  two  classes  of  cyclones 
to  deal  with.  No  rules  can  be  laid  down  in  the  abstract 
by  which,  given  a  cyclone  to-day,  he  can  calculate  where 
it  will  be  to-morrow.  But  by  experience  he  knows  that 
the  cyclones  which  form  near  Bermuda  run  a  totally 
different  course  from  those  which  form  over  Hudson's 
Bay,  and  he  can  generally  form  a  very  fair  estimate  of 
their  probable  motion. 

In  Great  Britain  it  is  evident  that  when  a  persistent 
spell  of  this  type  is  recognized  as  having  set  in,  the 
general  character  of  the  weather  and  direction  of  the 
wind  are  at  once  indicated.  The  forecaster  knows  that 


TYPES  AND  SPELLS   OF   WEATHER.  345 

the  cyclones  which  press  in  from  the  Atlantic  will  never 
get  past,  so  that  his  country  will  always  be  under  the 
influence  of  the  front  only  of  the  depressions.  All  that 
is  necessary  for  storm-warnings  is  to  watch  for  signs  of 
the  intensity  becoming  so  great  as  to  give  rise  to  a  gale. 
The  example  we  have  just  given  in  Figs.  64  and  65  is 
very  characteristic  of  a  gale  coming  on  entirely  from  in- 
crease of  intensity,  without  any  motion  of  a  cyclone.  This 
shows  the  value  of  any  indications  of  increasing  or  decreas- 
ing intensity  which  can  be  derived  from  any  source. 

An  inspection  of  the  illustrative  charts  will  show  that 
the  area  involved  is  so  large  that  it  is  hopeless  to  trace 
the  cyclones  as  a  whole,  but  that  usually  within  the  area 
of  the  British  telegraphic  reports,  and  always  somewhere, 
there  are  localities  to  the  east  or  north-east  where  the 
pressure  is  steady.  Over  the  Atlantic  great  variations 
occur,  and  the  forecaster  has,  therefore,  first  to  try  and 
discover  the  area  of  steady  pressure,  and  then  to  keep  a 
sharp  look-out  for  any  rapid  fall  of  the  barometer  over 
the  west  coast  of  Ireland,  which  would  produce  steep 
gradients  and  their  associated  gales.  When  once  a  frag- 
ment of  a  ring  of  steep  gradients  is  formed,  its  progress 
eastwards  must  be  traced  by  telegraph,  and  watch  must 
be  kept  that  there  is  no  giving  way  of  pressure  over 
Scandinavia.  Since  the  rate  of  progress  of  the  steep 
gradients  is  usually  slow  and  pretty  regular,  and  since, 
as  has"  beenj  shown  above,  the  direction  of  the  wind  with 
the  general  character  of  the  weather  is  subject  to  little 
uncertainty,  gales  'of  this  type  are  practically  forecast 
.with  almost  greater  success  than  any  other  class. 

The  forecaster  in  Central  Europe  is  not  so  fortunate. 


346  WEATHER. 

The  nature  of  the  changes  there  are  so  complex  and  sex 
ill  defined  that  he  can  scarcely  follow  them  after  they 
have  happened,  so  that  he  can  do  little  more  than  forecast 
generally  unsettled  weather  while  the  barometer  is  falling 
and  secondaries  are  forming  in  sympathy  with  the  great 
Atlantic  cyclones.  After  the  mercury  has  begun  to  rise, 
improving  weather  is  certainly  indicated. 

The  Kussian  forecaster  has  a  totally  different  task. 
He  recognizes  the  type,  and  knows  that  as  long  as  his 
anticyclone  lasts  there  is  no  fear  of  bad  weather.  We 
have  shown  that  there  is  always  some  isobar-,  in  this  case 
3OO  ins.  (763  mm.),  which  remains  nearly  stationary,  and 
he  has  to  find  this  out  in  each  case,  and  watch  for  any 
symptoms  of  a  serious  change. 

Thus  we  see,  as  the  foundation  of  all  synoptic  fore- 
casting, that  the  official  in  charge  of  the  central  bureau 
must  learn  by  experience  the  ways  of  cyclones  in  his  own 
country,  and  decide  each  case  on  its  own  merits  accord- 
ing to  the  best  of  his  judgment. 

The  property  of  any  type  of  weather  to  continue  for 
any  length  of  time  is  called  the  "  persistence "  of  that 
type.  Many  phases  of  weather  are  due  to  this  principle, 
and  for  forecasting  it  is  very  important  to  recognize  any 
signs  of  this  continuance ;  but,  as  the  indications  for  this 
type  are  the  same  as  for  any  other,  we  will  describe  the 
details  of  persistence  later  on. 

Then  as  to  signs  of  change.  This  type  may  merge 
insensibly  either  into  the  westerly  on  one  side,  or  the 
easterly  on  the  other,  the  latter  change  being  usually  the 
more  abrupt ;  but  it  is  not  possible  to  give  any  detailed 
description  of  symptoms  of  change. 


TYPES  AND  SPELLS  OF  WEATHER.  34:7 

WESTERLY  TYPE. 

In  this  type  the  permanent  belt  of  anticyclones  does 
not  extend  very  far  north,  and  pressure  decreases  steadily 
from  the  tropics  towards  the  north.  Under  these  circum- 
stances, cyclones  are  developed  on  the  north  side  of  the 
Atlantic  anticyclone,  which  roll  quickly  eastwards  along 
the  high-pressure  belt  and  usually  die  out  after  they 
have  become  detached  from  the  Atlantic  anticyclone  in 
their  eastward  course.  Their  intensity,  and  consequently 
the  weather  they  produce,  may  vary  almost  indefinitely. 
When  the  cyclones  are  formed  so  far  south  that  their 
centres  cross  Great  Britain,  and  are  of  moderate  size,  the 
intensity  is  usually  great,  and  severe  well-defined  storms, 
with  sharp  shifts  of  wind,  are  experienced.  These  occur 
most  frequently  in  spring  and  autumn,  and  are  the  most 
destructive  storms  which  traverse  Great  Britain. 

In  another  modification,  while  the  pressure  is  low  to 
the  north,  and  the  isobars  run  nearly  due  east  and  west, 
the  whole  of  the  arctic  area  of  low  pressure  surges  south- 
ward, with  an  exceedingly  ill-defined  cyclone,  bringing  a 
rim  of  steep  gradients  along  the  edge  of  the  Atlantic 
anticyclone,  and  across  Great  Britain,  in  a  manner 
analogous  to  the  phase  of  southerly  type  before  ex- 
plained. The  indications  then  are  for  rain  and  westerly 
gales,  with  very  little  shift  of  wind.  This  phase  belongs 
almost  exclusively  to  the  winter  months. 

But  the  commonest  modification  at  every  season,  and 
that  which  forms  about  seventy  per  cent,  of  European 
weather,  is  when  the  intensity  is  moderate,  and  the 
cyclone  paths  are  so  far  to  the  north  of  the  British 


348  WEATHER. 

Islands  that  the  wind  merely  backs  a  point  or  two  from 
the  south-west  as  the  cyclone  approaches,  and  veers  a 
point  or  two  towards  the  west  as  the  cyclone  passes,  the 
general  direction  of  the  wind  being  between  south-west 
and  west,  without  rising  to  the  strength  of  a  gale,  while 
rain  is  moderate  in  quantity. 

Sometimes  in  summer  a  prolongation  of  the  Atlantic 
anticyclone  covers  the  southern  portion  of  Great  Britain, 
and  distant  cyclones  of  small  energy  just  influence  the 
northern  countries  of  Europe.  Then  the  intensity  is 
too  small  to  develop  rain,  and  only  produces  cloud  in 
the  middle  of  the  day,  so  that  fine,  dry  weather  is  indi- 
cated, which  when  very  prolonged  may  give  rise  to 
drought.  This  is  by  far  the  commonest  of  all  weather 
types  in  temperate  regions,  and  occurs  at  every  season  of 
the  year. 

The  existence  of  this  type  in  Europe  is  sometimes 
associated  with  a  similar  phase  of  weather  in  the  United 
States.  That  is  to  say,  pressure  being  high  over  Mexico, 
cyclones  form  over  the  Kocky  Mountains,  and  then  pass 
along  the  line  of  the  Lakes  into  the  Atlantic.  To  this 
class  belong  almost  exclusively  the  cyclones  which  pass 
from  the  United  States,  over  the  Atlantic,  into  Europe. 
At  other  times,  a  persistent  anticyclone  may  cover  the 
American  continent,  and  the  whole  of  the  European 
system  of  cyclones  is  born  and  developed  in  mid- Atlantic. 

Before  we  give  more  details,  it  may  be  well  to 
exemplify  some  of  the  leading  features  of  this  type.  In 
Pigs.  68-71  we  therefore  give  charts  over  the  North 
Atlantic  and  Europe  for  the  four  days,  February  26  to 
March  1,  1865.  These  may  be  taken  to  represent  a  fair 


TYPES  AND   SPELLS  OF  WEATHER. 


34D 


100 


specimen  of  ordinary  broken  weather  in  Europe,  without 
sufficient  intensity  to  give  steep  gradients  and  severe 
gales.  In  all,  the  Atlantic  anticyclone  was  flanked  on 
the  west  by  another  over  the  American  continent,  and 

60  40  20  0  20  *0  GO 


80 


100  80  60  40  20  0 

FIG.  68.— Westerly  type  of  weather. 


40 


on  the  east  by  another  over  Central  Asia.  This  last 
only  appears  in  three  of  the  charts.  We  have  therefore 
to  deal  with  the  trade-wind  region  south  of  the  Atlantic 
anticyclone ;  the  cols  on  either  side  of  it ;  and  the  slope 
of  decreasing  pressure  which  extends  towards  the  Pole. 


60 


350 


WEATHER. 


We  shall  take  the  equatorial  region  first,  because  we 
want  to  show  the  nature  of  weather-changes  in  that  part 
of  the  world,  but  not  to  have  to  recur  to  the  subject 
again  till  the  end  of  this  chapter.  On  all  four  days  the 


20 


20 


60 


40 


20 


100 


60  40  20  0  i 

FIG.  69. — Westerly  type  of  weather,, 


40 


broad  features  of  tropical  pressure  distribution  are  the 
same ;  that  is  to  say,  the  type  of  weather  is  essen- 
tially constant.  But  in  details  no  two  days  are  alike, 
for  a  series  of  bends  in  the  isobars  denote  a  succession 
of  weather  changes,  some  of  whicli  eventually  have  an 


TYPES   AND   SPELLS   OF   WEATHER. 


351 


influence  on  Europe.  On  the  first  day,  February  26 
(Fig.  68),  the  isobar  of  29*9  ins., (760  mm.)  only  shows 
one  bend  northwards,  while  the  north-east  and  south-east 
trades  are  separated  by  a  calm  near  the  equator.  By 

100  80  60  40  20  0  20  40 


60 


ic!v" 

•''-^•f..  \ 

/  I ) 


0 


20 


100 


60  40  20  0  20 

FIG.  70. — Westerly  type  of  weather. 


40 


next  day  this  bend  had  become  more  pronounced,  and 
moved  a  little  more  to  the  north-east.  This  latter  motion 
is  very  interesting,  for  the  prevailing  wind  is  north- 
easterly, but  the  direction  of  the  wind  has  not  conformed 
to  the  bend  of  the  isobars  in  the  manner  which  might 
have  been  expected. 


S52 


WEATHER. 


On  the  third  day,  February  28  {Fig.  70),  very  great 
changes  have  occurred.  Under  the  col  which  lies  near 
Bermuda,  a  second  bend  has  made  its  appearance  so  a& 
to  greatly  modify  the  trade-wind  region  in  the  West 
Indies;  by  next  day  (Fig.  71)  this  bend  has  developed 
into  a  well-defined  cyclone  of  very  moderate  intensity,. 


FIG.  71. — Westerly  type  of  weather. 

which  moved  towards  the  north-east,  and  eventually 
affected  the  coasts  of  Great  Britain.  We  thus  see  that 
the  details  of  pressure-distribution  are  perpetually  changing 
in  this  region,  but  never  more  than  a  certain  amount. 
We  can  therefore  easily  understand  why  the  weather 
which  is  always  experienced  in  these  latitudes  is  described 


TYPES  AND   SPELLS   OF  WEATHER.  353 

as  generally  easterly,  variable  in  strength,  with  the 
weather  fine  or  showery  according  to  circumstances,  but 
never  following  the  cyclonic  sequence  of  the  temperate 
zone.  This  modified  alternation  of  weather  is  called  the 
fluctuation  of  its  type,  as  opposed  to  a  change  of  type, 
which  would  involve  a  totally  different  distribution  of 
pressure. 

From  this  digression  on  the  trade-winds  we  must  now 
return  to  the  cyclone-traversed  region  of  the  temperate 
zone  and  the  cols  of  the  more  tropical  parts  of  the  world. 
On  February  26  (Fig.  68),  we  find  a  fragment  of  a  large 
cyclone  over  Norway,  a  V  over  Great  Britain,  some 
complex  secondaries  over  the  Mediterranean,  and  an 
anticyclone  over  the  United  States.  Note,  however, 
three  innocent-looking  bends  on  the  north-west  edge  of 
the  Atlantic  anticyclone. 

By  next  day  the  Norwegian  cyclone  and  the  British 
V  have  fused  or  merged  into  an  irregular  cyclone  which 
covers  Scandinavia ;  while  the  Mediterranean  secondaries 
have  also  formed  a  new  cyclone,  and  a  corner  of  the 
Asiatic  anticyclone  just  appears  near  the  Black  Sea. 
Further  west,  the  three  bends  in  the  isobars  which  looked 
so  harmless  the  preceding  day  are  now  reduced  to  two, 
but  have  gained  intensity.  One  lies  to  the  south  of 
Iceland,  and  the  wedge  which  precedes  it  determines  the 
weather  for  this  day  in  Great  Britain.  The  other,  which 
is  less  intense,  lies  south  of  Newfoundland ;  but  the 
American  anticyclone  has  somewhat  retreated. 

By  next  day,  February  28  (Fig.  70),  the  Norwegian 
cyclone  had  nearly  died  out,  while  the  Atlantic  cyclone, 
with  its  associated  wedge,  had  travelled  eastwards  and 

2  A 


354  WEATHER. 

much  increased  in  intensity.  In  connection  with  this,  a 
mass  of  secondaries  had  developed  over  Germany  and 
Central  Europe  in  the  col  which  lay  between  the  Atlantic 
and  Asiatic  anticyclones.  Similar  changes  are  most  cha- 
racteristic of  European  weather  during  the  persistence 
of  this  type,  and  a  knowledge  of  them  is  of  the  utmost 
importance  in  forecasting.  The  cyclone  which  has  come 
in  from  the  Atlantic  is  'moving  and  will  continue  to 
move  towards  the  north-east,  and  so  far  it  might  be  said 
that  it  did  not  affect  the  forecasters  in  Central  Europe ; 
but  when  we  know  that  the  passage  of  the  depression 
will  develop  secondaries  and  bad  weather,  it  is  evident 
that  the  indirect  influence  of  the  Atlantic  cyclone  is 
very  great.  In  every  part  of  the  world  we  may  say  that 
the  passage  of  a  cyclone  in  the  temperate  zone  will 
develop  secondaries  in  the  tropical  col  over  which  it 
passes.  We  may  also  use  this  as  an  illustration  of  the 
fact  that  the  tracking  of  existing  cyclones  plays  but  a 
small  part  in  forecasting,  as  compared  with  the  larger 
question  of  detecting  influences  which  will  make  new 
cyclones  or  destroy  old  ones. 

The  col  nearly  over  Bermuda  had  developed  a  well- 
marked  inflection  near  the  West  Indies. 

Lastly,  by  the  morning  of  March  1  (Fig.  71)  all  these 
changes  had  somewhat  developed.  The  British  cyclone 
had  begun  to  fill  up,  and  the  European  secondaries  had 
much  diminished  in  intensity.  This  is  an  example  of 
what  we  have  already  mentioned  in  the  abstract — that  a 
cyclone  which  is  filling  up  is  decreasing  in  intensity,  and 
vice  versa.  In  mid- Atlantic,  the  bend  in  the  isobars  near 
Bermuda,  as  before  mentioned,  has  developed  into  a  small 


TYPES  AND   SPELLS  OF  WEATHER.  355 

cyclone,  which  lies  between  the  Atlantic  and  American 
anticyclones.  The  latter  has  moved  a  little  towards  the 
east. 

We  may  give  the  general  features  of  British  weather 
for  these  four  days  as  a  sample  of  the  type,  and  the 
reader  may  fill  up  those  in  any  other  country  for  himself. 
On  the  morning  of  the  26th,  the  weather  in  Great  Britain 
was  wet  and  broken  from  the  influence  of  the  V.  Next 
day  the  weather  was  beautifully  fine  from  the  wedge ; 
the  third  day,  wet  and  stormy — this  time  from  a  true 
•cyclone — and,  finally,  cold  and  fine  from  the  rear  of  the 
same  cyclone  on  the  fourth  day.  Similar  alternations 
of  weather  would  go  on,  with  endless  modifications,  so 
long  as  the  type  persisted.  From  this  we  see  the  con- 
trast which  the  westerly  type  presents  to  the  southerly 
one.  In  the  latter,  Great  Britain  was  constantly  exposed 
to  the  influence  of  the  fronts  only  of  cyclones ;  in  the 
former,  both  fronts  and  rears  develop  their  characteristic 
weathers.  We  gave  the  details  of  the  changes  of  tem- 
perature which  occurred  over  the  whole  of  Europe 
during  the  first  three  days  in  our  chapter  on  Heat  and 
€old. 

In  this  example  the  United  States  was  constantly 
under  the  influence  of  a  persistent  anticyclone,  and,  so 
far  as  it  goes,  this  shows  the  nature  of  a  type  of  weather 
in  that  country. 

We  can  now  easily  understand  the  following  particu- 
lars of  the  characteristic  weather  of  this  type. 

The  general  temperature  of  this  type  is  about  the 
average  of  the  season — a  little  warmer  in  front  of  the 
cyclones,  and  a  little  colcjer  in  rear.  In  winter,  however, 


35(5  WEATHER. 

a  great  prevalence  of  this  type  gives  an  open  season,  as 
the  high  wind  prevents  frost,  unless  the  cyclones  are  so 
far  north  that  the  influence  of  the  Atlantic  anticyclone 
is  felt. 

In  summer,  on  the  contrary,  if  the  type  be  intense, 
the  temperature  is  below  the  average,  from  the  excess  of 
cloud  hiding  the  sun. 

Another  important  consideration,  as  regards  tempera- 
ture, depends  on  the  position  of  the  normal  cyclone-path. 
The  difference  of  temperature  just  north  and  south  of  a 
cyclone-centre  is  very  marked,  so  that  when  cyclones 
pass  further  south  than  usual,  the  temperature  of  the 
region  lying  between  the  usual  and  actual  paths  is  greatly 
lowered. 

To  this  type  also  belongs  a  peculiar  class  of  warm, 
cloudy  anticyclones,  which  seem  to  be  associated  with 
cyclones  passing  to  the  far  north,  but  which  have  not 
yet  been  investigated. 

As  regards  damp,  wind,  and  weather,  the  most  notice- 
able feature  of  this  type  is  the  changeableness  of  all 
these  elements.  This  must  be  so,  because  the  rapidly 
moving  cyclones  bring  up  alternately  the  damp,  rainy, 
southerly  winds  and  the  dry,  cold,  northerly  currents  of 
their  fronts  and  rears  respectively. 

The  telegraphic  forecaster,  instead  of  thinking  how 
cyclones  are  going  to  die  out,  as  in  the  southerly  types, 
has  to  consider  along  what  paths  they  will  move.  No 
generalities  are  of  much  assistance ;  his  opinion  must  be 
formed  by  his  own  judgment,  and  from  experience  of 
cyclone-paths  in  his  own  country.  For  instance,  in  Great 
Britain,  he  can  often  tell  whether  the  centre  will  skirt 


TYPES  AND  SPELLS   OF   WEATHER.  357 

the  north-west  coasts  of  Scotland,  or  else  traverse  England 
on  its  way  to  Denmark. 

Dr.  J.  van  Bebber  has  classified  the  cyclone-paths  of 
Central  Europe  for  the  use  of  the  Deutsche  Seewarte, 
while  in  the  United  States  they  know  that  the  great 
majority  of  cyclone-paths  pass  along  the  line  of  the 
Lakes  and  St.  Lawrence  valley.  But,  in  spite  of  any 
classification,  we  must  never  forget  that  a  cyclone  may 
travel  in  nearly  any  direction,  and  for  that  reason  the 
knowledge  of  the  most  usual  paths  is  of  less  use  in  fore- 
casting than  in  explaining  the  climatic  peculiarities  of  a 
country. 

NORTHERLY  TYPE. 

The  special  feature  of  this  type  is  the  presence  of  a 
large  anticyclone  over  Greenland  and  the  arctic  portion 
of  the  Atlantic,  which  either  joins  the  Atlantic  anti- 
cyclone or  is  only  separated  from  it  by  a  col.  On  the 
east  side  of  this,  over  Europe  and  Russia,  lies  a  persistent 
area  of  low  pressure,  which  is  the  theatre  of  the  forma- 
tion of  an  incessant  series  of  cyclones,  while  innumerable 
secondaries  are  formed  over  Great  Britain  and  France. 
The  cyclones  either  move  eastwards,  or  else,  if  they  stand 
still,  surge  up  and  down  and  alter  their  shape  in  a  very 
peculiar  manner. 

This  is,  in  fact,  the  exact  converse  of  the  southerly 
type.  In  that,  Europe  was  persistently  under  the  in- 
fluence of  southerly  winds  and  cyclone-fronts ;  in  this,  it 
is  as  steadily  under  the  influence  of  northerly  winds  and 
cyclone-rears. 


358 


WEATHER. 


This  type  occurs  chiefly  in  the  winter,  spring,  and 
summer  ;  it  is  very  rare  in  the  autumn  months. 

On  the  American  side  of  the  Atlantic,  this  distribution 
of  pressure  exercises  a  profound  influence  on  the  general 
character  of  the  weather.  Instead  of  the  cyclones  finding 
an  easy  path  into  the  Atlantic,  their  eastward  progress 


DO  40 


O  2O 


FIG.  72. — Northerly  type  of  weather. 

is  checked  by  the  areas  of  high  pressure,  and  in  some 
instances  their  direction  is  even  reversed. 

For  instance,  in  Figs.  72-75  we  give  reductions  from 
the  United  States  maps  of  the  northern  hemisphere,  for 
March  22  to  25,  1878,  at  0.43  p.m.  Greenwich,  or  7.35 
a.m.  Washington  time.  In  all  the  Atlantic  high  pressure 


TYPES   AND   SPELLS   OF   WEATHER. 


359 


will  be  found  stretching  far  north,  till  it  nearly  meets 
another  anticyclone  lying  over  Greenland ;  and  in  all, 
relatively  low  pressure  will  be  found  both  over  Northern 
Europe  and  the  western  states  of  the  American  Union. 

On  March  22  (Fig.  72),  each  of  these  low  areas  con- 
tains a  cyclone,  one  over  Finland,  giving  northerly  winds 


FIG.  73. — Northerly  type  of  weather. 

and  cloudy  weather  over  Great  Britain  and  the  greater 
part  of  Europe ;  the  other  about  three  hundred  miles 
west  of  Newfoundland.  An  independent  cyclone  lies 
near  Florida,  and  a  col  separates  the  Atlantic  and  Green- 
land anticyclones. 

By  next  day  (Fig.  73),  though  the   centre   of  the 


360 


WEATHER. 


Finland  cyclone  has  hardly  changed  its  position,  the 
area  has  extended  westwards,  and  the  weather  over 
Western  Europe  becomes  rather  worse. 

Note  particularly  that  the  barometer  has  fallen  about 
three-tenths  of  an  inch  in  some  parts  of  England,  but 
owing  to  a  surge,  and  not  to  the  passage  of  a  cyclone. 


FIG.  74. — Northerly  type  of  weather. 

On  the  other  side  of  the  Atlantic,  the  Newfoundland 
cyclone  has  moved  westwards,  joined  the  Florida  cyclone, 
and  so  extended  its  area  as  to  cover  the  whole  of  the 
northern  states.  This  is  the  reverse  of  any  we  have 
seen  before.  The  Atlantic  anticyclone  has  enlarged,  and 
projects  further  north. 


TYPES  AND   SPELLS  OF  WEATHEK. 


361 


By  midday  of  the  24th  (Fig.  74),  the  Finland  cyclone 
has  lost  any  definite  shape,  while  another  centre  has 
formed  over  the  Carpathians,  and  a  complicated  system 
of  secondaries  over  Western  Europe.  The  whole  is  most 
typical  of  this  kind  of  weather. 

We  referred  to  this  chart  in  our  chapter  on  Squalls, 


FIG.  75. — Northerly  type  of  weather. 

for  out  of  the  complex  bends  in  the  isobars  which  we  see 
over  England  and  France  developed  a  V-shaped  de- 
pression of  great  intensity,  a  squall  in  which  capsized  the 
British  man-of-war  Eurydice  almost  within  sight  of  port. 

The  American  cyclone  has  moved  towards  the  south- 
west, and  is  now  centred  over  the  New  England  states. 


362  WEATHER. 

It  has  also  slightly  diminished  in  size,  but  increased  in 
intensity,  probably  under  the  action  of  the  anticyclone 
which  lies  in  the  north-west. 

Lastly,  on  the  chart  for  March  25  (Fig.  75),  we 
see  that  the  two  centres  of  the  European  cyclone  have 
moved  as  if  they  were  revolving  round  each  other,  or 
round  a  common  centre,  while  the  whole  level  has  risen, 
and  the  secondaries  have  much  diminished  in  com- 
plexity. 

With  these  changes,  and  the  rise  of  the  barometer, 
the  weather  over  Great  Britain  and  Western  Europe  has 
much  improved,  but  the  wind  retains  its  prevailing 
northerly  set. 

Our  illustration  certainly  represents  weather-changes 
of  exceptional  complexity,  but  still  it  shows  all  the  more 
forcibly  the  impossibility  of  applying  numerical  calcu- 
lations either  to  the  motions,  the  winds,  or  any  other 
phenomena  of  a  cyclone. 

This  is  equally  evident  when  we  look  on  the  other 
side  of  the  Atlantic.  The  cyclone  there  has  reversed 
its  direction  and  now  gone  towards  the  north-east.  Besides 
this,  the  intensity  has  still  further  increased  so  as  to  give 
worse  weather  over  Canada,  New  Brunswick,  and  Nova 
Scotia,  while  one  secondary  projects  towards  Bermuda, 
and  another  in  the  direction  of  Iceland. 

So  long  as  this  type  continues  the  sequence  of  weather 
at  any  station  is  tolerably  simple  in  Great  Britain.  As 
the  barometer  falls,  the  wind  veers  towards  the  north- 
east, with  a  hard,  cloudy  sky ;  wind  and  rain  according 
to  the  intensity,  with  an  increase  of  temperature ;  and 
then  the  sky  clears,  the  wind  backs  by  north  towards  the 


TYPES  AND  SPELLS  OF  WEATHER.  363 

north-west,  and  the  air  gets  colder  as  the  mercury  begins 
to  rise. 

But  during  the  whole  continuance  of  this  type,  the 
general  northerly  set  of  the  wind  and  the  peculiarly 
hard  sky  are  never  lost,  and  numerous  secondaries  will 
give  rise  to  many  puzzling  contradictions  between  the 
movement  of  the  barometer  and  the  severity  of  the 
weather. 

From  this  it  is  manifest  that  the  general  temperature 
of  the  type  must  be  below  the  average,  and  the  air  must 
be  also  dry  from  the  prevalence  of  northerly  winds. 

During  the  persistence  of  this  sequence  of  weather, 
all  European  forecasters  have  to  solve  a  problem  exactly 
the  converse  of  that  which  was  presented  to  them  by  the 
southerly  type.  Then  they  looked  westwards  for  the 
daily  arrival  of  cyclones,  and  eastwards  for  any  symptoms 
of  a  change  of  type.  Now  they  look  eastwards  for  a 
daily  formation  of  new  depressions,  and  westwards  for 
any  signs  of  decreasing  pressure  over  Ireland  which  would 
be  the  forerunner  of  a  different  type  of  atmospheric 
circulation. 

EASTERLY  TYPE. 

In  this  type  the  sequence  of  weather  and  cyclone- 
motion  turns  round  the  presence  of  a  persistent  anti- 
cyclone over  Scandinavia,  which  profoundly  modified  the 
motion  of  depressions  which  come  in  from  the  Atlantic. 
The  Atlantic  anticyclone  is,  of  course,  always  there ;  but 
a  col,  which  is  formed  between  it  and  the  Scandinavian 
high  pressure,  crosses  Europe  and  impresses  a  very 


36  4<  WEATHER. 

definite  character  on  the  weather-changes.  When  cyclones 
coming  in  from  the  Atlantic  meet  this  col,  they  are 
either  arrested  in  their  course,  and  remain  brooding  over 
the  Bay  of  Biscay,  or  else  they  pass  through  the  col  in 
a  south-easterly  direction.  In  rare  cases  cyclones  are 
formed  on  the  southern  side  of  the  Scandinavian  anti- 
cyclone, with  their  centres  over  Southern  Europe  or  the 
Mediterranean  Sea,  and  these  often  move  towards  some 
point  of  west.  Nothing  can  show  more  clearly  than  this 
the  value  of  type-groups  in  determining  the  probable 
course  of  any  cyclone.  In  the  abstract,  a  cyclone  may 
go  in  any  direction,  and  in  all  the  European  classes  we 
have  so  far  examined  they  always  move  towards  some 
point  of  east ;  but  in  this  type  of  pressure-distribution 
only  we  may  sometimes  look  for  depressions  which  travel 
westwards. 

This  type  occurs  at  all  seasons  of  the  year,  though 
it  is  most  frequent  in  winter  and  spring,  and  most  rare 
in  autumn.  In  Great  Britain  it  often  persists  for  two 
or  three  weeks  consecutively,  and  gives  rise  to  destructive 
easterly  gales.  Nearly  one-half  of  the  wrecks  on  the 
British  coast  are  due  to  gales  of  this  class.  No  direct 
connection  can  be  traced  between  the  occurrence  of  this 
type  in  Europe  and  any  particular  phase  of  weather  in 
the  United  States  or  Canada. 

But  before  we  go  into  details,  we  may  illustrate  the 
nature  of  this  type  by  an  actual  example.  In  Figs. 
76-79,  we  give  large  charts  of  a  considerable  portion  of 
the  northern  hemisphere  for  the  four  days,  February 
25-28,  1875,  at  about  8  a.m.,  Greenwich.  In  all,  an  area 
of  high  pressure  rests  over  Scandinavia,  while  the  Atlantic 


TYPES  AND   SPELLS   OF   WEATHER. 


365 


anticyclone  reaches  so  far  north  as  to  suggest  some 
features  of  the  northerly  type.  The  col  of  low  pressure 
below  these  two  anticyclones  is  the  theatre  of  cyclone 
activity,  and  we  will  now  describe  how  the  weather  in 
Western  Europe  was  affected  by  these  changes.  On  the 
morning  of  February  25  (Fig.  76),  we  find  the  Scandi- 


60  40 


FIG.  76. — Easterly  type  of  weather. 

navian  anticyclone  almost  meeting  a  wedge  of  high 
pressure  stretching  northwards  from  the  Atlantic  anti- 
cyclone to  Greenland.  The  pressure  for  several  days 
previous  had  belonged  to  the  northerly  type,  with  an 
anticyclone  over  Greenland,  which  had  now  drifted  east- 
wards and  joined  the  Scandinavian  anticyclone.  To  the 


366 


WEATHER. 


south  of  this  at  least  three  cyclones  are  found :  one  over 
the  Azores,  another  at  the  entrance  to  the  English 
Channel,  the  third  over  Italy.  These  must  all  be  treated 
as  belonging  to  the  same  system,  as  they  are  all  formed 
in  the  same  pit  of  low  pressure.  The  weather,  of  course, 
is  bad  all  over  France,  Germany,  and  Italy.  The  American 


FIG.  77. — Easterly  type  of  weather. 

reports  are  meagre,  but  point  to  the  existence  of  a  cyclone 
in  Lower  Canada. 

By  next  day  (Fig.  77),  the  Scandinavian  anticyclone 
has  increased  in  height,  while  the  Atlantic  one  has 
retreated  nearly  to  its  usual  position.  The  Italian 
cyclone  has  moved  a  little  to  the  north-east,  while  that 


TYPES   AND   SPELLS   OF   WEATHER. 


367 


in  the  Bay  of  Biscay  has  apparently  moved  a  very  little 
to  the  south-west,  and  so  far  absorbed  the  Azores  depres- 
sion that  the  latter  has  become  degraded  into  a  secondary. 
Here  we  have  the  same  fusion  of  cyclones  which  we  have 
seen  in  all  the  other  types,  combined  with  the  stationary 
character  which  is  so  peculiar  to  this  class  of  weather. 


FIG.  78. — Easterly  type  of  weather. 

Across  the  Atlantic  an  intense  secondary  has  formed  over 
New  Brunswick,  while  another  shallow  one  has  pushed 
itself  into  the  col  between  the  Scandinavian  and  Atlantic 
cyclones. 

On  ^February  27  (Fig.  78)  these  changes  have  made 
further  progress.      Though  the  general  position  of  the 


368 


WEATHER. 


European  area  of  low  pressure  has  not  materially  altered, 
the  cyclones  which  lie  within  it  have  decreased  in  com- 
plexity, though  a  new  depression  has  formed  in  a  col 
between  the  Azores  and  the  Canaries.  The  two  American 
secondaries  have  fused  into  one  large  primary,  and  a 
large  col  covers  the  Central  Atlantic. 


FIG.  79. — Easterly  type  of  weather. 

Lastly,  on  the  28th  (Fig.  79),  while  the  Scandinavian 
anticyclone  has  diminished  in  height  and  area,  the  Atlantic 
anticyclone,  on  the  contrary,  has  increased  no  less  than 
04  inch  (10  mm.)  in  height,  and  much  increased  in  size. 
The  size  and  intensity  of  the  European  low  pressure  has 
diminished,  but  its  components  are  more  complex;  so 


TYPES  AND   SPELLS   OF  WEATHER.  369 

that  while  weather  has  improved  over  Great  Britain,  it 
is  worse  in  many  parts  of  France  and  Italy.  Across  the 
Atlantic,  all  we  can  say  is  that  where  one  large  cyclone 
was  yesterday,  there  are  now  two  secondaries :  one  intense 
over  Nova  Scotia,  another  slight  in  the  Atlantic  col. 
This  is  one  of  the  numerous  cases  where  it  is  impossible 
to  trace  the  exact  history  of  pressure-changes. 

The  general  character  of  the  weather  in  Great  Britain 
during  the  persistence  of  this  type  is  very  well  marked. 
The  sky  is  usually  black,  and,  even  if  there  is  a  certain 
amount  of  blue  overhead,  the  horizon  has  a  peculiar 
black,  misty  look,  popularly  known  as  an  "eastern  haze." 
This  is  quite  different  from  the  misty  horizon  of  a  calm 
day  in  the  westerly  type,  and  is  associated  with  the 
peculiar  bitter  feel  of  an  east  wind.  A  well-known 
saying  is — 

"  When  the  wind  is  in  the  east, 
It's  good  for  neither  man  nor  beast ;  " 

and  this  is  certainly  no  exaggeration. 

This  is  the  most  striking  illustration  we  can  have  of 
the  general  principle  that  no  instrumental  records  can 
take  the  place  of  verbal  description.  We  might  find  two 
north-east  winds  recorded  automatically,  of  exactly  the 
same  velocity  and  temperature — one  on  the  northern  side 
of  a  cyclone  of  the  westerly  type,  the  other  at  the  edge 
of  the  Scandinavian  anticyclone  in  the  easterly  type. 
Head  mechanically,  they  might  be  taken  to  be  identical, 
while  practically  they  are  very  different.  Unfortunately 
we  can  give  no  explanation  of  the  malignant  nature  of 
true  east  winds. 

The  temperature  is  generally  low,  but  more  variable 

2  B 


370  WEATHER. 

than  during  the  northerly  type.  This  is  because  the 
cyclone-centres  sometimes  get  so  far  east  as  to  bring  up 
a  breath  of  southerly  wind,  which  is  speedily  driven  back 
by  a  new  irruption  of  pressure  from  Scandinavia. 

The  wind  is  always  from  some  point  of  east,  with  less 
tendency  to  back  towards  the  north  than  during  the 
continuance  of  the  northerly  type,  and  generally  keeping 
between  north-east  and  south-east.  The  contrast  between 
this  and  the  westerly  type  will  be  strikingly  evident  if 
we  look  back  at  Figs.  71-73,  and  note  that  they  refer  to 
the  same  three  days  of  the  year,  February  26-28,  as 
our  last  three  (Figs.  77-79).  By  selecting  these  dates  on 
different  years,  all  diurnal  and  seasonal  variations  are 
equalized,  and  the  entire  difference  of  wind  and  weather 
is  solely  due  to  difference  of  type. 

Forecasting  during  the  persistence  of  this  type  pre- 
sents the  greatest  difficulties,  especially  in  Western 
Europe.  Though  the  general  character  of  the  weather- 
sequence  may  be  sufficiently  obvious,  still  there  is  the 
utmost  uncertainty  as  to  the  paths  of  cyclones.  When 
these  come  in  from  the  Atlantic,  we  have  no  means  of 
saying  whether  they  will  pass  through  the  col  in  a  south- 
easterly direction,  or  whether  they  will  be  deflected  to  a 
north-easterly  course.  In  addition  to  this,  the  motion 
of  cyclones,  in  whatever  direction,  is  so  irregular  that  the 
forecasters  doomed  to  frequent  failure. 

The  signs  of  persistence  are  chiefly  such  as  may  be 
derived  from  watching  the  position  of  the  Scandinavian 
anticyclones,  and  the  continuance  of  low  pressure  to 
the  west  of  Ireland.  The  signs  of  change,  on  the  con- 
trary, turn  round  any  diminution  of  pressure  in  Sweden, 


TYPES  AND  SPELLS   OF   WEATHER.  371 

or  the  appearance  of  high  pressure  far  north  in  the 
Atlantic. 

The  four  great  types  of  weather  which  we  have  now 
sketched  are  capable  of  being  divided  more  minutely 
into  sub-types ;  but  these  would  vary  so  much  for  different 
countries,  that  they  cannot  be  detailed  in  this  work.  All 
that  we  can  do  here  is  to  note  the  universality  of  the 
principle,  and  the  properties  of  weather  which  the  exist- 
ence of  types  explains  so  readily. 

Although  we  have  already  mentioned  the  great  prin- 
ciples of  weather-changes  which  are  designated  by  the 
terms  "intensity,"  "fluctuation,"  "persistence,"  "recur- 
rence," and  "dependence"  of  type  more  or  less  inci- 
dentally, it  may  be  well  to  add  a  few  remarks  here  on 
all  of  them,  beginning  with — 

INTENSITY. 

We  have  already  explained  the  term  "  intensity  "  as 
applied  to  single  cyclones,  and  shown  both  how  it  is 
measured  by  the  gradients  and  how  it  influences  the 
weather,, 

But  "  intensity  of  type  "  denotes  the  character  of  a 
sequence  of  weather  to  which  the  epithet  of  "  broken " 
would  be  applied.  Broken  weather  is  found  by  synoptic 
charts  to  be  the  product  either  of  small  quick-moving 
cyclones  which  only  exist  for  a  very  short  time,  or  of 
numerous  secondaries  ;  in  contradistinction  to  the  weather 
produced  by  large  low-gradient  cyclones,  moving  slowly 
and  lasting  for  some  days,  which  would  be  associated 
with  more  settled  weather. 


372  WEATHER. 

The  relation  between  these  two  kinds  of  intensity  is 
analogous  to  that  between  long,  single,  heavy  gusts,  and 
numerous  short  puffs  of  wind;  both  are  symptoms  of 
great  atmospheric  disturbance,  though  of  a  different  kind 
in  each  case. 


FLUCTUATION. 

The  word  "  fluctuation "  is  applied  to  that  limited 
alternation  of  a  general  distribution  of  pressure  which 
occurs  every  day  all  over  the  world.  In  the  tropics, 
where  pressure-distribution  is  unchanged  for  months, 
fluctuation  is  chiefly  confined  to  small  modifications  of 
intensity,  which  make  the  weather  a  little  better  or  a 
little  worse,  according  to  circumstances.  In  semi-tropical 
countries  fluctuation  is  much  larger,  usually  from  the 
formation  of  secondaries,  though  the  general  type  does 
not  materially  change.  In  the  temperate  zone,  on  the 
contrary,  we  have  not  only  enormous  fluctuation  of  type, 
but  also  complete  alteration  of  the  type  itself.  The 
classification  of  phenomena  called  fluctuation  is  of  the 
greatest  value  in  handling  questions  of  weather-sequence, 
as  it  enables  us  to  separate  that  which  is  incidental  from 
that  which  is  essential  to  any  type. 

PERSISTENCE. 

The  word  "  persistence "  describes  that  prominent 
feature  of  remaining  pretty  stationary  which  charac- 
terizes all  pressure-distribution  over  large  areas.  This 
is  always  concurrent  with  a  persistence  of  appropriate 


TYPES   AND   SPELLS   OF   WEATHER.  373 

weather,  and  in  this  property  of  types  we  find  the  explana- 
tion of  many  phenomena  of  weather  and  of  many  popular 
prognostics. 

For  instance,  in  Great  Britain,  an  interval  of  cold 
weather  in  winter  may  be  produced  by  the  persistent 
influence  of  either  the  northerly  or  easterly  type ;  or,  if 
only  for  two  or  three  days,  from  the  wedge-shaped  area 
of  high  pressure  between  two  cyclones.  So  also  a  drought 
may  be  induced  either  by  a  persistent  anticyclone,  or 
else  by  cyclone-centres  far  north,  when  the  intensity  is 
slight,  while  long-continued  rain  may  accompany  almost 
any  persistent  type  if  the  gradients  be  steep. 

Then,  as  to  weather-prognostics.  It  is  a  well-known 
saying,  that  "When  grouse  come  down  into  the  farm- 
yards it  is  a  sign  of  snow."  The  birds  are  driven  down 
in  search  of  food  by  the  excess  of  snow  already  existing 
on  the  moors,  and  so  far  the  prognostic  would  refer  to  the 
past  rather  than  to  the  future ;  but,  by  the  principle  of 
persistence,  the  type  which  has  already  given  so  much 
snow  may  be  expected  to  continue  for  some  time,  and 
therefore  more  snow  may  be  expected. 

In  Germany  there  is  a  proverb,  "  Fresh  snow,  fresh 
cold,"  which  holds  good  for  the  same  reason. 

Similarly,  the  prognostics,  "  When  a  river  rises  with- 
out any  rain  having  fallen,  bad  weather  may  be  expected," 
or  "  Irregular  tides  are  signs  of  rain,"  have  a  significance 
for  the  future,  though  both  are  caused  by  past  bad  weather 
at  a  distance;  for  the  persistent  type  will  almost  cer- 
tainly, sooner  or  later,  bring  more  bad  weather  over  the 
place  of  observation. 

On  the  same  principle,  the  prognostic,  "  Breakers  in 


874  WEATHER. 

shore  without  wind  are  a  sign  of  storm,"  holds  on  the  east 
coast  as  well  as  on  the  west,  but  for  a  different  reason. 

On  the  west  coast,  the  breakers  have  sometimes  run 
on  ahead  of  the  cyclone  which  raised  them ;  but  on  the 
east  coast  this  does  not  occur,  as,  practically,  all  cyclones 
move  towards  some  point  of  east. 

Nevertheless,  though  the  storm  which  raised  the 
waves  has  never  affected  the  place  where  they  occur, 
still  it  is  extremely  probable  that  another  of  the  same 
series  will  do  so ;  therefore  the  prognostic  is  good, 
though  less  certain  than  on  the  west  coast. 

It  is  also  manifest  that  the  principle  of  persistence 
has  an  important  bearing  on  forecasts.  Unfortunately, 
though  such  types  are  common,  it  is  not  yet  possible  to 
define  any  certain  indications  of  change  from  one  to 
another.  One  sign  of  persistence  may,  however,  be  men- 
tioned which  rarely  fails. 

Sometimes  a  type  apparently  fails  for  a  day  or  two, 
bat  then  is  re-established  with  great  intensity.  When 
this  occurs,  its  continuance  for  a  considerable  time  may 
safely  be  predicted.  For  instance,  with  the  easterly 
type  a  small  cyclone  frequently  passes  rather  far  to  the 
east,  and  the  wind  shifts  to  the  south-west  with  increased 
warmth;  but  when  this  dies  out  the  easterly  type  is 
re-established  in  full  force.  In  these  cases  the  appearance 
of  the  weather  is  sometimes  very  characteristic,  for,  though 
the  wind  is  west,  the  look  is  that  of  an  east  wind,  and 
so  obvious  is  this  that  the  people  say  "  that  the  east 
wind  is  not  gone  yet." 


NIVEK. 

Ni^ 

TYPES  AND   SPELLS   OF   WEATHER.  875 

BECURRENCE. 

We  have  already  explained  the  tendency  of  certain 
kinds  of  weather  to  recur  about  the  same  date  every  year 
so  fully  in  our  chapter  on  Seasonal  Variations,  that  it  is 
unnecessary  here  to  do  more  than  allude  to  that  great 
principle  of  meteorology.  We  shall,  however,  better 
understand  now  how  the  recurrence  of  weather  is  the 
secondary  product  of  the  recurrence  of  a  certain  type  of 
pressure-distribution;  and  that  to  be  a  true  periodicity 
of  cold,  for  instance,  it  is  not  only  cold  in  the  abstract, 
but  cold  of  the  same  type  which  must  recur  about  the 
same  date  in  most  years. 

DEPENDENCE. 

By  "dependence"  of  type  or  weather  is  meant  the 
supposed  connection  between  the  occurrence  of  any  par- 
ticular type  at  one  season  of  the  year,  and  the  consequent 
occurrence  of  it  or  of  another  type  at  another  season. 

For  instance,  there  is  a  common  saying  in  Great 
Britain,  that  if  easterly  winds  prevail  about  the  time 
of  the  spring  equinox,  then  a  great  preponderance  of 
easterly  winds  may  be  expected  during  the  summer. 
Put  into  the  language  of  synoptic  charts  and  types,  this 
means  if  the  easterly  type  happens  to  prevail  about  the 
23rd  of  March,  then  there  will  be  a  tendency  of  that 
type  to  occur  more  often  than  usual  in  the  course  of  the 
summer. 

Again,  in  most  temperate  countries,  hot  summers  are 
popularly  supposed  to  be  followed  by  cold  winters,  and 


376  WEATHER. 

the  latter  are  thought  to  depend  in  some  way  on  the 
former.  This  is  much  more  difficult  to  express  in  synoptic 
language,  for  heat  and  cold  are  not  always  produced  by 
the  same  causes,  and,  unless  the  same  type  of  summer 
is  followed  by  the  same  type  of  winter,  the  apparent 
relation  of  the  two  seasons  is  illusory. 

The  same  conception  of  the  dependence  of  one  season 
on  the  other  is  found  in  the  tropics.  H.  F.  Blanford 
has  found  that  in  India  there  is  an  apparent  dependence 
or  sequence  of  the  summer  wet  season  on  the  preceding 
winter  rains. 

At  present  we  can  do  little  more  than  note  such  a 
relationship  of  seasons,  and  cannot  say  whether  there  is 
even  such  a  dependence  at  all.  The  older  weather-lore 
seems  to  have  been  founded  partly  on  observation,  partly 
on  an  intuitive  belief  in  the  general  balance  of  nature. 
In  the  main,  the  course  of  nature  is  constant;  if  the 
summer  is  hotter  than  usual,  a  cold  winter  is  required  to 
restore  equilibrium,  and  so  on  for  any  other  phenomenon 
of  weather. 

The  middle  stage  of  meteorological  investigation 
seeks  to  find  proof  of  such  relation  by  comparing  statistics 
of  rain  at  different  seasons.  Here,  of  course,  the  great 
difficulty  is  to  be  certain  whether  all  the  rains  which  we 
compare  are  really  of  the  same  type. 

The  latest  phase  of  thought  would  look  for  some 
connection  or  sequence  between  the  forms  and  intensities 
of  atmospheric  eddies.  All  we  can  do  is  to  note  the  facts 
for  future  research ;  and  to  remark  that  at  the  present 
time  no  use  can  be  made  of  dependence  of  type  in 
practical  weather-forecasting. 


TYPES  AND  SPELLS  OF  WEATHER.  377 

CHANGE  OF  TYPE. 

So  far  we  have  supposed  well-defined  specimens  of. 
each  type,  but  in  practice  we  meet  with  many  transitional 
forms.  Thus  the  southerly  type  may  merge  by  insensible 
gradations  into  either  the  easterly  or  westerly,  but  in  no 
case  can  it  grow  into  the  northerly.  Similarly  the 
westerly  type  may  approximate  on  either  side  towards 
the  southerly  or  northerly,  but  never  jump  suddenly  into 
the  easterly.  In  like  manner  the  northerly  and  easterly 
types  can  only  merge  into  those  next  to  themselves  on 
either  side,  but  never  into  their  opposites.  This  is 
obvious  when  we  reflect  that  the  types  are  determined,  by 
the  surrounding  anticyclones,  and  that  a  slight  shift  of 
one  of  these  latter  may  modify  the  type  very  materially 
on  either  side,  while  a  change  to  an  opposite  type  would 
involve  a  total  rearrangement  of  pressure  over  the  whole 
northern  hemisphere. 

In  a  few  cases  we  have  been  able  to  point  out  signs  of 
an  impending  change  of  type,  but  unfortunately  the 
forecaster  is  often  confronted  by  very  sudden  altera- 
tions in  the  whole  distribution  of  pressure  over  the 
northern  hemisphere.  Future  research  may  perhaps 
some  day  lead  to  the  detection  of  more  certain  symptoms 
of  change,  though  at  present  we  can  say  but  little. 

NORTH-EAST  MONSOON. 

But  perhaps  the  nature  of  European  types  will  be 
more  readily  comprehended  if  we  give  some  illustrations 
of  the  Indian  monsoons.  These  will  be  very  valuable 


378 


WEATHER. 


both  as  showing  weather-features  of  a  totally  different 
character  from  any  which  we  have  hitherto  examined,  and 
as  explaining  the  connection  between  the  fluctuations  of 
weather  in  the  tropics  and  the  more  variable  changes  of 
the  temperate  regions.  In  Figs.  80  and  81  we  therefore 
give  isobaric  and  isothermal  charts  for  January  4  and  5, 


-p.m.  4.35  6.jj 

FIG.  80. — North-east  monsoon;  great  cold. 

1878,  at  about  6-30  p.m.,  Calcutta  time — that  is,  during 
the  season  of  the  north-east  monsoon  in  the  Indian  Ocean. 
These  charts  commence  in  longitude  40°  east  of  Green- 
wich, where  our  Atlantic  maps  left  off,  and  so  continue 
on  the  same  projection  our  survey  of  the  world  60°  further 
east. 


TYPES  AND  SPELLS  OF  WEATHER. 


370 


On  both  days  we  find  an  anticyclone,  exceeding  31*0 
ins.  in  height,  resting  over  Tartary,  to  the  east  of  Lake 
Aral.  North  of  this,  pressure  slopes  away  towards  the 
Arctic  Ocean ;  southwards  the  pressure  falls  away  to 
the  equator.  In  fact,  this  anticyclone  is  probably  the 
counterpart  of  the  Atlantic  anticyclone ;  while  the  low 


FIG.  81. — North-east  monsoon  j  great  cold. 

pressure  over  Southern  India  corresponds  to  the  trade- 
wind  slope,  which  we  also  saw  in  the  Atlantic.  The  most 
noticeable  feature  in  both  these  charts  is  the  persistence 
of  the  central  Asiatic  anticyclone  and  of  the  southern 
slope  of  low  pressure,  while  the  northern  elope  is  more 
variable ;  just  as  we  saw  in  the  Atlantic. 


380  WEATHER. 

The  wind  circulates  round  the  anticyclone  in  the 
usual  manner;  but  note,  however,  that  the  winds  over 
Lower  Bengal  are  from  north-west,  not  from  north-east, 
as  might  have  been  expected.  This  is  due  to  a  small 
permanent  depression  near  the  mouth  of  the  Ganges,  that 
cannot  be  shown  on  so  small  a  map.  As  this  general 
distribution  of  pressure  lasts  all  through  the  winter 
months,  we  see  that  the  north-east  monsoon  is  the  exact 
representative  of  the  trade-winds  of  the  Atlantic;  only 
that  the  result  of  Asia  being  a  land  area  is  that  the 
easterly  winds  have  a  far  greater  extension  northwards 
than  over  the  ocean. 

We  have  already  referred  to  the  temperature-charts 
in  our  chapter  on  Heat  and  Cold.  All  that  we  need  do 
here  is  to  call  attention  to  the  great  cold — 30°  Fahr. — 
near  the  centre  of  the  anticyclone. 

The  sky  was  blue  and  clear  at  almost  every  station 
in  India  on  both  the  days  in  question.  What  most  con- 
cerns us  here  is  to  note  the  limited  amount  of  fluctuation 
over  India,  and  the  somewhat  irregular  nature  of  the 
winds,  relative  to  the  isobars,  in  that  country.  For 
instance,  in  Fig.  80  the  isobar  of  30*0  in.  is  to  the  north 
of  Calcutta,  while  on  the  following  day  (Fig.  81)  it  is  a 
short  distance  to  the  south  of  that  city.  This,  of  course, 
would  have  been  associated  with  a  rise  of  the  barometer, 
though  the  general  character  of  the  monsoon  would  not 
have  been  affected.  This  is,  in  fact,  precisely  analogous 
to  the  fluctuation  of  the  isobars  which  we  saw  over  the 
Atlantic  to  the  south  of  the  great  permanent  anticyclone. 
Temperature  varies  in  a  similar  manner,  for  the  isotherm 
of  60°  has  been  somewhat  deflected  on  the  second  day  by 
the  increasing  pressure. 


TYPES  AND  SPELLS   OF  WEATHER.  381 

As  these  charts  are  very  fair  specimens  of  any  others 
during  the  persistence  of  this  monsoon,  we  see  that  the 
task  of  the  Indian  forecaster  would  be  comparatively 
simple.  For,  though  a  limited  fluctuation  of  the  general 
distribution  of  pressure  takes  place  from  day  to  day,  the 
amount  never  exceeds  a  moderate  quantity,  and  still  less 
is  the  whole  character  of  the  weather  ever  altered  in  the 
manner  which  we  have  seen  in  more  northern  latitudes. 


THE  SOUTH-WEST  MONSOON. 

The  general  character  of  the  south-west  monsoon  will 
be  best  illustrated  by  giving  first  a  typical  sample  of  two 
consecutive  days,  and  then  by  making  some  remarks  on 
the  whole  system  of  Indian  weather. 

In  Figs.  82  and  83  we  therefore  give  charts  for  June 
17  and  18,  1881,  over  India  and  Central  Asia,  at  about 
6*30  p.m.,  Calcutta  time.  These  may  be  considered  as 
typical  of  the  distribution  of  pressure  in  those  countries 
during  the  summer  months,  just  after  the  burst  of  the 
south-west  monsoon  of  the  Indian  Ocean.  In  Bengal  they 
relate  to  the  time  when  the  hot  season  has  just  begun  to 
give  place  to  the  rains. 

In  both  maps  we  see  an  oval  isobar  enclosing  pressure 
less  than  29'4  ins.  round  Lahore,  in  the  Punjab ;  and  in 
both  an  isotherm  of  100°  Fahr.  (38°  C.),  nearly  con- 
terminous with  that  isobar.  This  depression  is  usually 
more  distorted  by  secondaries  than  on  these  two  days. 

The  winds  blow,  on  the  whole,  round  the  lowest 
pressure  in  the  usual  manner,  being  from  west  or  south- 
west to  south,  and  from  north-east  or  east  on  the  northern 


382 


WEATHER. 


side  of  the  low  pressure.  The  few  wind-arrows  which  our 
diagrams  admit  of  will,  however,  show  that  the  relation  of 
wind-direction  to  isobars  is  not  so  constant  as  in  higher 
latitudes.  For  instance,  the  winds  in  Lower  Bengal  are 
more  from  the  north-west  than  the  general  laws  of  wind- 
rotation  would  have  indicated. 


p.m.         4.35 
FIG.  82. — South-west  monsoon ;  great  heat. 

The  weather  was  cloudy  or  overcast  at  almost  every 
station,  with  rain  at  several,  and  blue  sky  at  only  one  on 
either  day;  but  all  the  rain  is  either  of  the  secondary 
or  of  the  non-isobaric  type,  and  cannot  be  located  by 
looking  at  the  charts. 

It  is  very  interesting  to  contrast  this  weather  with 


TYPES  AND   SPELLS  OF  WEATHER. 


383 


that  of  the  north-east  monsoon.  In  the  latter  there  is  a 
difference  of  one  inch  of  pressure  and  100°  of  temperature 
between  Central  Asia  and  India ;  in  the  former  only  six- 
tenths  of  an  inch  of  pressure  and  40°  of  temperature.  In 
the  cold  monsoon  there  is  scarcely  anything  but  blue 
sky ;  while  with  the  warm  south-west  wind  the  heavens 
are  almost  entirely  covered  by  clouds. 


&?0 


700° 


p.m.         4.35 
FIG.  83.— South- west  monsoon;  great  heat. 

An  important  but  difficult  question  will  immediately 
present  itself  as  to  the  relation  which  the  low  summer 
pressure  over  India  bears  to  the  equatorial  bert  of  low 
pressure  over  the  rest  of  the  world. 

In  these  last  two  maps  we  no  longer  find  any  arrange- 


384  WEATHER. 

ment  similar  to  that  which  occurs  in  the  Atlantic.  The 
persistent  pit  of  low  pressure  over  Scinde  and  the  North- 
Western  Provinces  of  British  India  is  probably  the 
representative  of  the  equatorial  belt  of  low  pressure 
which  constantly  covers  the  Atlantic  about  10°  north 
latitude.  Here,  however,  the  lowest  point  is  nearly  in 
latitude  30°  north ;  but  we  know  that  there  is  no  lower 
pressure  between  it  and  the  equator. 

The  greatest  difference  is  in  the  absence  of  an  anti- 
cyclone north  of  the  equatorial  low-pressure  belt.  In 
Fig.  82  there  is  a  very  narrow  arm  of  high  pressure 
between  the  two  isobars  of  28*8,  and  just  the  fragment  of 
an  anticyclone  stretching  over  from  the  north-east  of 
Siberia.  This  latter  is  very  persistent  over  that  region  at 
this  season  of  the  year,  but  the  difficult  point  is  to  deter- 
mine the  relation  of  the  Indian  low  pressure  to  the  low 
pressure  which  usually  lies  over  European  Eussia  in  the 
summer  months.  This  we  are  unable  to  give.  But  we 
may  notice  that  a  somewhat  similar  phenomenon  appears 
on  a  smaller  scale  over  Mexico  in  the  summer.  Then  the 
Atlantic  anticyclone,  and  another  one  in  the  Pacific,  about 
the  same  latitude,  form  a  col  over  Mexico ;  pressure  at 
the  same  time  is  persistently  low  over  the  United  States 
and  also  over  Central  America.  Then  the  distribution  of 
pressure  over  the  whole  American  continent  has  some 
analogies  to  that  over  Asia  at  the  same  season  of  the  year. 

Many  meteorologists  have  contended  that  this  circular 
persistent  depression  over  Upper  India  should  be  con- 
sidered as  a  stationary  cyclone ;  but  the  author's  researches 
have  conclusively  proved  that  if  we  take  cloud-forms  as 
a  test,  none  of  the  true  monsoon  rains  partake  of  the 


TYPES   AND   SPELLS   OF   WEATHER  385 

character  of  a  primary  cyclone-front — roost  of  the  rain 
falls  from  cumuloform  clouds — while  that  in  front  of  a 
Bengal  or  any  other  cyclone  seems  to  grow  out  of  the  air. 
This  Indian  depression  seems  to  be  somewhat  analogous 
to  the  pit  of  low  pressure  which  covers  the  North  Atlantic 
during  the  winter  months.  Neither  are  cyclonic,  but  both 
are  the  theatre  of  atmospheric  disturbance.  The  former 
only  breeds  thunderstorms  and  secondaries ;  the  latter, 
well-developed  primary  cyclones. 

We  have  already  described,  in  our  chapter  on  Non- 
isobaric  Rains,  the  remarkable  character  and  unknown 
origin  of  the  rain  in  this  south-west  monsoon ;  and  how, 
without  any  marked  change  in  the  shape  or  position  of 
the  isobars,  the  dry,  hot  south-west  breezes  are  suddenly 
converted  into  wet  and  stormy  winds.  There  can  be 
little  doubt  that  the  source  of  this  change  is  to  be 
found  in  the  upper  currents  which  feed  the  south-west 
surface-winds,  but  the  subject  is  too  obscure  to  be 
discussed  in  this  work. 

What  mostly  concerns  us  here  is  the  nature  of  the 
limited  fluctuation  of  isobars  and  weather  over  India ;  for 
this  is  typical  of  the  origin  of  the  modified  day  to  day 
weather-changes  in  the  tropics  as  opposed  to  extreme 
changes  of  the  temperate  zone.  There  is  little  fluctuation 
in  the  two  charts  we  have  given  in  Figs.  82  and  83 ;  but 
the  bend  in  the  isobar  of  29*6  below  Calcutta  is  less  pro- 
nounced on  the  second  than  on  the  first  day.  This  would 
be  associated  with  a  slight  improvement  in  the  weather 
on  the  second  day. 

Sometimes,  however,  larger  changes  take  place  during 
the  continuance  of  this  monsoon.  Once  or  twice  during 

2o 


386  WEATHER. 

the  rainy  season — June  to  October — the  pit  of  low  pressure 
near  Lahore  stretches  further  east,  and  becomes  less 
pronounced,  and  less  distorted  by  secondaries.  H.  F. 
Blanford  has  shown  that  this  fluctuation  is  associated 
with  that  period  of  drier  weather  which  is  known  as  a 
"  break  in  the  rains." 

We  may  correct  here  a  popular  error  that  during  the 
rainy  season  in  any  part  of  the  tropics  it  rains  all  or  even 
every  day.  Sometimes,  no  doubt,  rain  may  fall  twenty- 
nine  out  of  thirty  days,  or  for  forty-eight  hours  without 
cessation ;  but  there  are  always  periods  of  less  intensity 
and  of  more  intermittent  showers. 

Returning  to  India  in  particular,  as  the  season  gets 
on,  small  secondary  cyclones  occasionally  form  over  the 
Bay  of  Bengal,  and,  advancing  nearly  northwards,  strike 
land  on  the  Orissa  coast,  and  continue  their  course  undis- 
turbed by  mountain  or  valley  till  they  reach  the  great 
chain  of  the  Himalayas.  In  this  these  secondaries  contrast 
in  a  marked  degree  with  the  great  primary  cyclones  which 
form  on  the  Bay  of  Bengal  at  the  change  of  the  monsoon. 
These  latter  almost  invariably  break  up  when  the  centre 
reaches  the  land.  The  main  characteristic  of  the 
secondaries  is  light  wind  with  torrential  rain ;  as  much  as 
fifteen  inches  of  rain  has  been  collected  within  twenty- 
four  hours  during  the  passage  of  one  of  these  small 
depressions. 

Occasionally,  during  the  month  of  May,  primary 
cyclones  of  considerable  intensity  develop  over  the  Bay 
of  Bengal,  and  move  towards  some  point  of  the  west, 
usually  to  north-west ;  and  again  in  October,  as  the 
south-west  monsoon  gives  way  to  that  from  the  north- 


TYPES  AND   SPELLS  OF   WEATHER.  387 

east,  cyclones  of  very  great  intensity  form  in  the  same 
Bay,  and  these  too  are  propagated  towards  the  west  or 
north-west.  In  either  case  the  coasts  exposed  to  their 
influence  experience  very  bad  weather,  with  rain  and 
wind  of  hurricane  force  in  the  October  cyclones. 

From  this  very  brief  sketch  of  monsoon  weather  we 
may,  however,  learn  that  the  persistent  seasonal  weather 
in  the  tropics  is  exactly  analogous  to  the  persistent  types 
of  weather  which  appear  periodically  in  temperate  regions. 
Both  are  primarily  caused  by  the  distribution  of  pressure, 
but  the  changes  which  only  occur  once  a  year  in  the 
tropics  take  place  at  short  and  at  irregular  intervals  in 
higher  latitudes,  while  between  the  two  extremes  we  find 
an  intermediate  series  of  recurrent  types  which  are 
neither  so  regular  as  in  the  tropics  nor  so  uncertain  as  in 
Western  Europe.  In  both  the  differences  of  weather  from 
day  to  day  are  due  to  the  fluctuation  of  the  type,  or  to 
small  alterations  either  in  the  shape  or  intensity  of  the 
depressions  which  form  in  the  low-pressure  areas. 

We  also  see  problems  of  forecasting  totally  different 
from  any  which  present  themselves  in  the  European  or 
American  offices,  and  had  we  been  able  to  give  a  greater 
number  of  examples,  we  should  have  found  the  truth  of 
the  general  principle  that  no  mechanical  rules  can  be 
laid  down  as  to  the  probable  path  of  a  cyclone,  or  the 
fluctuation  of  a  type,  but  that  the  forecaster  who  knows 
the  ways  of  the  barometric  movements  in  his  own  country 
can  usually  form  a  very  good  opinion  as  to  their  future 
progress. 

It  may  be  advantageous  here  to  pause  a  moment  and 
survey  the  general  aspect  of  the  great  problem  of  weather 


388  WEATHER. 

as  it  is  now  presented  to  us.  We  have  seen  over  a  large 
portion  of  the  world  the  same  seven  forms  of  isobars  con- 
stantly reproduced,  though  with  details  greatly  modified, 
not  only  by  the  type,  but  still  further  by  the  size  and 
intensity,  by  the  time  of  day,  the  season  of  the  year,  and 
also  by  local  causes. 

But  though  these  sources  of  variation  prevent  our 
writing  down  on  a  chart  more  than  the  general  character 
of  the  weather  under  any  isobars,  their  classification, 
grouping,  and  co-ordination  each  in  its  proper  place, 
enable  us  to  distinguish  the  essential  from  the  more 
accidental  features  of  weather. 

And  when  we  come  to  watch  the  ceaseless  changes  of 
isobars,  we  see  that  sometimes  cyclones  disappear  so 
quickly  that  within  twenty-four  hours  no  trace  of  their 
existence  is  to  be  found,  while  at  other  times  the  same 
cyclone  may  exist  for  weeks  together.  Then  also  we  see 
that  a  cyclone  may  either  remain  stationary,  or  move  in 
almost  any  direction  with  a  very  wide  range  of  velocity ; 
and  we  learn  the  still  more  curious  phenomenon  of  that 
fusion  of  one  or  more  cyclones  into  a  single  system  which 
so  often  makes  it  impossible  to  track  the  paths  of 
depressions. 

We  also  see  very  clearly  how  the  old  idea  that  a 
cyclone  is  necessarily  a  destructive  storm  is  no  longer 
tenable  ;  and  that  we  must  adapt  ourselves  to  the  concep- 
tion that  a  cyclone  is  an  eddy  of  very  variable  intensity, 
always  rainy  and  always  surrounded  by  a  very  definite 
rotation  of  air,  though  the  force  of  the  wind  may  vary 
from  a  zephyr  to  a  hurricane.  When  we  talk  of  cyclonic 
weather  we  must  use  descriptive  epithets  such  as  mode- 


TYPES  AND   SPELLS   OF  WEATHER.  389 

rate,  intense,  etc.,  to  denote  the  general  force  of  the 
wind. 

It  is  also  manifest  from  the  great  scale  on  which 
changes  of  pressure-distribution  take  place,  that  there  is 
some  greater  cause  at  work  behind  them  than  any  local 
developments  of  heat  or  rain.  This  cause  is  undoubtedly 
the  general  circulation  of  the  atmosphere  from  the  hot 
equator  to  the  cold  Poles  ;  though  doubtless  temperature 
and  precipitation  have  a  modifying  effect  on  the  greater 
changes.  If  the  earth  were  surrounded  by  a  vapourless 
atmosphere,  cyclones  and  anticyclones  would  undoubtedly 
be  formed,  though  not  the  same  as  those  with  which  we 
are  so  familiar. 

Now  that  we  know  what  weather  is,  we  may  consider 
how  far  it  can  be  forecast  more  or  less  in  advance. 


390  WEATHER 


CHAPTER  XIV. 

FORECASTING  FOB  SOLITARY  OBSERVERS. 
NATURE  OF  THE  PROBLEM. 

A  COMPREHENSIVE  view  of  weather- science  divides  itself 
into  three  problems — one  direct  and  two  inverse.  The 
direct  problem  of  weather  is  to  explain  by  mechanical 
causes  the  origin  and  nature  of  all  the  complicated 
phenomena  of  wind  and  weather  which  present  themselves 
to  our  senses,  and  the  nature  of  tue  sequence  of  weather- 
changes.  This  we  have  already  partially  done  in  the 
preceding  pages.  The  inverse  problem  of  meteorology  is, 
given  a  portion  of  a  sequence  of  the  weather,  to  tell  what 
is  going  to  follow.  The  morning  is  fine,  but  now  cirrus 
begins  to  form,  and  the  mercury  has  begun  to  fall — what 
weather  is  coming  ?  Last  night  a  cyclone  lay  over 
Ireland,  this  morning  it  covers  Wales — what  will  the 
weather  be  over  Great  Britain  for  the  rest  of  the  day  ? 

These  two  illustrations  point  at  once  to  a  natural 
subdivision  of  the  questions  of  forecasting:  the  best 
that  a  single  observer  can  do,  who  has  his  eyes  to  look  at 
the  appearance  of  the  sky  and  any  instruments  at  his 


FORECASTING  FOR  SOLITARY   OBSERVERS.  391 

disposal ;  and  the  best  that  a  meteorologist  can  do,  who  is 
seated  in  a  central  bureau,  with  abundant  telegraphic 
intelligence  for  many  miles  round  the  country  for  which 
he  has  to  issue  forecasts,  so  as  to  enable  him  to  construct 
synoptic  charts  at  such  intervals  as  he  may  think  neces- 
sary. The  latter  doubtless  represents  the  highest  develop- 
ment of  which  forecasting  is  capable  ;  but  the  former  can 
never  be  superseded  for  use  among  sailors,  fishermen,  and 
shepherds.  For  this  reason  we  will  discuss  them  in 
separate  chapters,  and  we  will  take  the  problem  of  a 
solitary  observer  first,  as  it  is  the  older  and  the  more 
generally  useful.  We  shall  only  attempt  to  give  general 
principles,  and  not  to  go  into  all  the  details  for  any  one 
country. 

PROGNOSTICS. 

We  have  already  gone  very  fully  into  the  subject  of 
prognostics,  and  pointed  out  both  the  reasons  for  their 
success  as  well  as  for  their  failure.  When  we  come  to 
look  at  all  that  has  been  done,  we  see  that,  on  the  whole, 
we  have  not  been  able  to  develop  the  practical  utility  of 
prognostics  very  materially,  though  we  have  been  able  to 
place  the  whole  branch  of  the  subject  on  a  scientific 
basis. 

The  most  valuable  addition  of  recent  times  to  weather- 
lore  is  undoubtedly  in  the  methodical  observation  of 
cirrus  clouds.  The  recognition  of  cirrus  as  a  sign  of  rain 
is  as  old  as  meteorology,  but  the  deductions  which  can  be 
made  from  the  direction  of  the  motion  of  the  upper 
clouds  are  quite  of  modern  date.  No  absolute  test  can 


392  WEATHER. 

be  given  for  the  discrimination  of  fine  weather  from 
dangerous  cirrus  beyond  the  general  surroundings  and 
experience  of  the  observer ;  but  Ley  has  shown  the 
importance  of  noting  by  eye  the  velocity  of  the  cirrus, 
because  rapid-moving  cirrus  is  a  much  worse  sign  of  the 
weather  than  slowly  moving  cloud.  This  is  probably  one 
of  the  most  important  advances  which  has  been  made. 


THE  BAROMETER. 

We  propose  rather  in  this  chapter  to  deal  with  the 
value  of  the  indications  which  the  barometer  can  afford 
to  a  solitary  observer,  and  especially  to  explain  why  the 
indications  of  that  instrument  so  often  fail. 

Why  do  we  sometimes  have  rain  with  a  rising  or 
steady  barometer,  and  why  is  the  weather  sometimes  fine 
with  a  falling  barometer?  Then,  again,  why  do  we 
sometimes  experience  a  heavy  gale  with  only  a  slight 
fall  of  the  mercury,  while  at  other  times  the  barometer 
will  fall  very  low  without  any  unusual  amount  of  wind  ? 

These  apparent  anomalies  in  the  indications  of  the 
barometer  occur  all  over  the  world,  and  those  in  each 
country  must  be  explained  by  reference  to  the  meteorology 
of  the  place.  Though  we  shall  draw  our  illustrations 
from  Great  Britain  only,  the  principles  which  we  shall 
lay  down  are  of  universal  application.  In  no  branch  of 
the  subject  shall  we  find  synoptic  charts  more  indis- 
pensable, for  without  them  no  explanation  could  ever 
have  been  afforded  of  irregular  barometric  fluctuations. 


FOEECASTING  FOR  SOLITARY  OBSERVERS.        303 

GENERAL  INDICATIONS. 

The  preceding  chapters  will  have  sufficiently  ex- 
plained the  reasons  for  what  we  may  call  the  generally 
correct  indications  of  the  barometer.  We  can  now  readily 
understand  why  the  rapid  rise  in  rear  of  a  cyclone  in- 
dicates unsettled  weather,  and  the  gradual  rise  of  an 
incipient  anticyclone  settled  fine  weather ;  also  why  the 
steady  barometer  of  a  persistent  anticyclone  indicates 
dry  seasonable  weather,  and  the  rapid  fall  of  an  oncoming 
cyclone  presages  storm  and  rain.  All  these  indications 
of  the  barometer  can  be  detected  by  intermittent  obser- 
vations, or,  in  fact,  by  merely  looking  occasionally  at  the 
instrument. 

The  author  has,  however,  discovered  that  we  can 
sometimes  utilize  the  greater  refinements  of  self- registered 
barographs  to  deduce  some  knowledge  of  the  future  force 
of  the  wind  from  flexures  in  the  recorded  curves.  These 
deductions  are  of  the  more  value  now  that  efficient  baro- 
graphs are  so  cheap  as  to  be  within  the  reach  of 
everybody. 

AUTHOR'S  KULES  FOR  INFERRING  FROM  A  BAROGKAM 
WHETHER  A  GALE  IS  GOING  TO  INCREASE  OR 
DECREASE. 

The  principle  on  which  the  author's  rules  are  founded 
depend  on  what  is  called  the  "  direction  of  curvature  "  of 
a  curve.  In  the  lower  portion  of  Fig.  84,  the  portion  of 
the  trace  near  the  letter  A  has  its  hollow  turned  upwards, 
and  is  called  convex,  relative  to  the  base  line.  A  little 


394 


WEATHER. 


further  down,  near  the  figures  14,  the  curve  is  hollowed 
downwards,  and  would  be  called  concave.  The  other  half 
of  the  curve  is  convex  almost  throughout.  From  this  we 
see  that  both  convexity  and  concavity  are  independent  of 


30  I 


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297 
295 
293 
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299 
297 
295 
293 
291 
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v            / 

^^-^^  ^/ 

FIG.  84. — Gradients,  and  flexure  of  barogram. 

whether  the  mercury  is  rising  or  falling,  and  also  of  the 
rapidity  of  their  rise  or  fall. 

If  the  barometer  change  at  a  uniform  rate,  either 
upwards  or  downwards,  it  is  evident  that  the  resulting 
trace  will  be  a  straight  line,  either  rising  or  falling,  and 
it  does  not  the  least  matter  how  rapid  the  rise  or  fall  is. 
If,  however,  the  rate  of  fall  changes  with  diminishing 
pressure,  then  the  curve  will  become  convex  or  concave, 


FORECASTING  FOE  SOLITARY   OBSERVERS. 


395 


according  as  the  rate  increases  or  decreases.  For  instance, 
suppose,  as  in  Fig.  85,  F,  that  the  barometer  fell  two- 
tenths  of  an  inch  between  one  and  two  o'clock,  and 
another  two-tenths  between  two  and  three  o'clock,  the 
resulting  barographic  trace  would  be  a  straight  descending 
line,  like  s;  if  in  the  second  hour  the  mercury  fell  three- 
tenths  of  an  inch  instead  of  only  two-tenths,  the  resulting 
trace  would  be  a  convex,  like  #;  while  if  it  only  fell 

2          3  Hours      I  23, 


In. 


\ 


FIG.  85. — Illustrating  the  origin  of  convex  and  concave  barograms. 

one- tenth  in  the  second  hour,  the  trace  would  be  concave, 
as  a. 

If  we  define  the  barometric  rate  as  the  number  of 
hundredths  of  an  inch  which  the  mercury  moves,  either 
up  or  down,  per  hour,  the  above  may  be  put  in  this  form. 

With  a  falling  barometer,  the  trace  is  convex  for  an 
increasing  rate,  concave  for  a  decreasing  one.  A  glance 
at  Fig.  85,  R,  will  show  that  for  a  rising  barometer  the 
converse  is  the  case ;  for  when  the  rise  is  greater  the 
second  than  the  first  hour,  the  trace  is  concave,  as  in  A ; 
but  when  less,  then  convex,  as  at  x ;  and  this  result  may 
be  stated  as  follows.  With  a  rising  barometer,  the  trace 


396  WEATHER. 

is  convex  for  a  decreasing  rate,  concave  for  an  increasing 
one.  This  is  the  reverse  of  what  happens  with  a  falling 
barometer.  Now,  the  simplest  and  commonest  case  of 
barometric  change  occurs  when  the  centre  of  a  cyclone 
drifts  past  a  station ;  the  fall  of  the  barometer  is  then 
proportional  to  the  steepness  of  the  gradients.  When 
steeper  gradients  approach,  the  barogram  will  become 
convex ;  when  slighter  gradients  arrive,  the  curve  will  be 
concave.  The  converse  holds  good  for  a  rising  baro- 
meter :  when  steeper  gradients  approach,  the  cur\7e  is 
concave ;  when  slighter,  then  convex. 

Now,  as  the  force  of  the  wind  is  proportional  to  the 
steepness  of  the  gradients,  we  find  that  the  direction  of 
curvature  of  a  barogram  tells  us  whether  a  gale  is  going 
to  get  worse  or  otherwise,  because  we  can  tell  if  the 
gradients  are  becoming  steeper  or  otherwise.  We  must 
be  very  careful  to  remember  that,  though  a  rapid  rate  of 
fall  is  in  a  general  way  a  worse  sign  of  weather  than  a 
moderate  one,  the  indications  deduced  from  the  curvature 
of  a  barographic  trace  depend  on  the  variation  of  the 
rate,  and  not  on  the  rate  itself.  For  instance,  in  Fig.  84, 
the  top  part  of  which  gives  the  isobars  over  Great  Britain 
on  November  14, 1875,  at  8  a.m.,  the  crossed  line  denotes 
the  direction  of  the  cyclone,  and  an  unsymmetrical 
arrangement  of  the  steepest  gradients  with  reference 
to  the  centre  is  very  obvious. 

To  get  the  barographic  section  of  a  cyclone,  or  to  find 
out  what  curve  the  propagation  of  the  depression  would 
leave  on  a  recording  instrument,  we  have  to  draw  a  line 
across  any  portion  of  the  plan,  as  shown  on  a  synoptic 
chart,  parallel  to  the  path  of  the  cyclone,  and  then,  by 


FORECASTING  FOR  SOLITARY  OBSERVERS.  397 

measuring  the  distance  in  time  between  any  two  con- 
secutive isobars,  we  arrive  at  the  flexures  of  the  trace. 
For  the  sake  of  simplicity,  we  will  suppose,  in  the  first 
instance,  that  we  are  stationed  exactly  on  the  line  of  the 
path  of  the  cyclone,  so  that  the  centre  will  pass  over  us. 
By  this  we  make  the  line  of  section  of  the  cyclone 
coincide  with  the  line  of  gradients,  which  is  not  the  case 
in  any  other  portion  of  the  depression. 

In  the  lower  part  of  Fig.  84  we  give  such  a  section 
of  the  cyclone,  sketched  in  the  upper  portion,  along  the 
line  A  B.  The  position  of  A  and  B  are  obverted  in  the 
section,  so  as  to  read  from  left  to  right  like  an  ordinary 
barogram.  Then  we  see  that  as  the  cyclone  approached 
the  gradients  got  steeper,  so  that  the  rate  of  barometric 
fall  increased,  and  therefore  the  trace  was  convex ;  during 
this  period  the  gale  got  worse.  After  a  time,  as  the  ring 
of  steep  gradients  passed,  and  the  slighter  gradients 
in  front  of  the  centre  approached,  the  rate  of  fall  of 
the  mercury  decreased  and  the  trace  became  concave, 
though  still  going  downwards.  The  gale  moderated 
somewhat  during  this  time.  The  passage  of  the  centre 
marked  the  turn  of  the  barometer ;  but  as  the  distance 
between  each  consecutive  isobar  increased  regularly  after 
29*5  inch,  the  resulting  barogram  was  convex  after 
that  level.  The  actual  curve  for  the  day,  as  given 
at  Stonyhurst,  which  lay  almost  in  the  line  of  the 
centre,  differs  only  slightly  from  this.  Thus  we  see  that 
the  normal  barographic  trace  in  a  cyclone  is  simply  the 
reflection  of  the  typical  shape  of  isobars  in  that  kind  of 
depression,  and  that,  moreover,  to  a  single  observer  the 
direction  of  curvature — that  is,  the  convexity  or  concavity 


398  WEATHER. 

of  a  barogram — enables  him  to  tell  whether  more  or  less 
steep  gradients  are  approaching,  and  therefore  whether  a 
gale  is  going  to  get  better  or  worse.  There  is,  however, 
one  limitation  which  considerably  detracts  from  the  value 
of  this  deduction.  If  the  line  of  section  of  the  cyclone 
which  passes  over  the  observer  is  not  square  to  the 
isobars,  the  relative  distance  between  any  two  consecutive 
isobars  is  no  longer  a  measure  of  the  gradients.  For 
instance,  if  the  cyclone  in  Fig.  84  had  passed  over  an 
observer  anywhere  on  the  line  c  G,  his  trace  from  o  to  E 
would  have  been  concave,  because  c  D  is  a  shorter  line 
than  D  E.  But  all  the  time  he  is  getting  into  a  region 
of  steeper  gradients,  as  measured  square  to  the  isobars, 
and  therefore  the  criterion  of  increased  gradients  fails. 
But  if  a  concave  need  not  be  an  absolute  test  of  de- 
creasing gradients,  a  convex  trace  can  never  fail  to  indi- 
cate steeper  gradients  with  a  falling  barometer.  This 
may  be  readily  seen  by  considering  the  nature  of  con- 
centric lines. 

Conversely,  with  a  rising  barometer,  we  see,  in  Fig.  84, 
that  from  E  to  G  the  barogram  will  be  concave,  though  the 
gradients  are  decreasing ;  but  under  no  possible  conditions 
could  a  convex  trace  fail  to  indicate  a  decreasing  gradient. 
The  author's  rule  is,  then,  as  follows : — Assuming  that  the 
force  of  a  gale  is  proportional  to  the  gradients,  a  convex 
barogram  is  always  bad  with  a  falling,  and  good  with  a 
rising  barometer ;  a  concave  trace  is  sometimes  a  good 
sign  with  a  falling,  and  not  always  a  bad  indication  with 
a  rising  barometer. 

This  rule,  of  course,  involves  the  supposition  that  the 
motion  of  the  barometer  is  solely  due  to  the  propagation 


FORECASTING  FOB  SOLITARY  OBSERVERS.     399 

of  isobars  over  the  observer,  but  in  practice  much  more 
complicated  changes  sometimes  occur. 

For  instance,  in  a  very  common  class  of  gale  belong- 
ing to  what  we  have  described  as  the  southerly  type  of 
weather,  a  cyclone,  after  arriving  near  the  British  coasts, 
remains  stationary,  but  increases,  maybe,  half  an  inch  in 
depth.  The  fall  of  the  barometer  which  then  occurs  at 
any  station  is  no  longer  of  the  same  kind  as  that  which 
we  have  just  examined,  and  the  flexure  of  the  trace  is 
determined  by  other  considerations.  The  direction  of 
curvature  would  then  depend  on  any  variation  of  the  rate 
of  deepening,  not  on  the  motion  of  the  cyclone. 

For  instance,  suppose  a  stationary  cyclone  which 
began  to  deepen  from  increasing  intensity — if  the  rate  of 
deepening  was  constant,  the  trace  would  be  a  straight 
descending  line ;  if  the  rate  increased,  the  curve  would 
be  convex  ;  if  it  decreased,  concave. 

But,  as  we  know  that  the  deepening  of  a  cyclone 
means  increased  intensity,  we  may  look  on  a  decrease  of 
that  rate  as  a  favourable  sign,  and  therefore  the  indica- 
tions of  the  relation  of  curvature  to  weather  would  remain 
good.  The  complications  which  arise  from  a  deepening 
or  shallowing  moving  cyclone  need  not  be  discussed  here, 
but  it  is  important  to  notice  the  two  distinct  causes  of 
barometric  change — the  passage  of  a  moving  cyclone,  and 
the  deepening  of  a  stationary  one. 

APPARENT  FAILURES  OF  THE  BAROMETER. 

So  far  we  have  dealt  with  what  may  be  called  the 
regular  movements  of  the  barometer,  that  is  to  say,  move- 


400  LEATHER. 

merits  which  are  associated  with  or  followed  by  the 
weather  which  was  anticipated.  But  we  must  now  explain 
certain  cases  in  which  the  weather  and  the  barometer  do 
not  seem  to  be  connected  in  the  ordinary  manner,  and 
show  how,  in  spite  of  apparent  anomalies,  the  same 
general  principles  of  meteorology  hold  throughout. 

ClRRUS   BEFORE   THE    BAROMETER. 

In  the  regular  course  of  isobaric  movements,  there  is 
one  case  in  which  cloud,  and  sometimes  rain,  forms  before 
the  barometer  begins  to  fall,  though  almost  immediately 
the  mercury  turns  downwards  and  falls  fast  with  in- 
creasing rain.  Thf,s  happens  just  in  front  of  the  crest  of 
a  wedge,  and  it  is  for  this  reason  that  in  the  diagram  of 
wind  and  weather  in  wedges  which  we  gave  in  Fig.  7,  we 
placed  the  word  "  halo  "  partly  in  front  of  the  line  of  the 
crest.  This  is  quite  common  in  Great  Britain,  and  it 
often  causes  comment  that  the  cirrus  begins  to  form 
before  the  barometer  indicates  the  approach  of  rain. 
Here,  in  fact,  the  sky  speaks  first,  but  not  so  soon  as  the 
isobars.  If  any  morning  a  British  forecaster  saw  a  wedge 
lying  over  Ireland,  and  blue  sky  was  reported  from  the 
east  of  England,  he  could  safely  forecast  that  cirrus  would 
appear  in  the  course  of  the  day,  before  the  barometer 
began  to  fall. 

On  the  other  hand,  the  author  has  discovered  that,  in 
the  tropics  especially,  the  ominous  sunsets  which  precede 
a  hurricane  are  developed  often  twenty-four  hours  before 
any  appreciable  depression  is  formed  anywhere ;  and  of 
course  squalls  and  secondaries  have  threatening  skies  as 


FORECASTING  FOR  SOLITARY  OBSERVERS.     401 

their  forerunners  without  any  definite  barometric  indica- 
tions. We  may  lay  it  down  as  a  general  rule  that  when 
the  sky  threatens  while  the  barometer  says  nothing, 
something  bad  is  coming ;  but  whether  thunder,  squall, 
or  gale  depends  on  circumstances. 

BAIN  WITH  A  KISING  BAROMETER  AND  AN  EAST  WIND. 

Kain  with  a  rising  barometer  and  an  east  wind  is  so 
common  in  England  that  Admiral  Fitzroy  engraved  it  on 
the  scales  of  his  barometers  as  an  exception  to  the  general 
rule  that  the  mercury  fell  for  rain.  No  explanation  was, 
however,  attempted,  and,  in  fact,  could  not  have  been 
given,  in  the  then  state  of  meteorology.  The  author  has 
made  a  large  number  of  unpublished  observations  on  the 
subject,  and  he  finds  a  singular  uniformity  in  the  isobaric 
conditions  under  which  this  apparent  anomaly  appears. 

In  every  case  which  he  has  examined,  the  rain  with 
a  rising  barometer  was  associated  with  a  peculiar  phase 
of  the  northerly  type  of  weather.  This,  as  we  explained 
in  the  last  chapter,  is  a  type  or  spell  of  weather  in  which 
pressure  remains  constantly  high  to  the  north  and  north- 
west of  Great  Britain,  while  cyclones  form  over  France  or 
Germany. 

The  character  of  this  phase  will  be  best  understood 
by  means  of  an  actual  example.  In  Fig.  86  we  give  a 
copy  of  a  barogram  in  London  on  April  20,  1877,  and 
underneath  the  appropriate  portion  we  have  marked  the 
time  during  which  rain  fell. 

Now,  at  first  sight  this  might  seem  opposed  to  all  we 
have  said  before  as  to  the  nature  of  cyclone-barogranas. 

2  D 


402 


WEATHER. 


Instead  of  a  well-marked  fall  of  the  mercury,  with  rain 
near  the  lowest  portion,  we  see  a  remarkably  uniform 


Inch. 

JO.O 

^9-5 

20.  0 

6.  a.m.        Noon.           6.  p.m.         Mic 

^^^**^~~ 

29-4-77 

London 

R   a 
E. 

E»ei^M0 

/      71 

Wind. 

FIG.  86. — Rain  with  rising  barometer  and  east  wind. 

trace,  in  which  the  diurnal  variation  of  the  barometer  is 
very  clearly  marked.      A  slight  general  rise,  however, 


PIG.  87. — Charts  to  illustrate  rain  with  rising  barometer  and  east  wind. 

occurred  in  the  afternoon,  and  rain  fell  from  about  3  to 


FORECASTING  FOR  SOLITARY  OBSERVERS.     403 

9  p.m.,  while  the  wind  remained  with  little  change  from 
north-east. 

In  Fig.  87  we  give  charts  at  8  a.m.  and  6  p.m.  on  that 
day,  so  that  the  changes  of  pressure  as  shown  by  the 
isobars  may  be  readily  apprehended.  In  both  charts  the 
edge  of  an  anticyclone  covers  the  north  of  Scotland,  but 
the  ill-defined  area  of  low  pressure  which  lay  over  France 
and  the  English  Channel  in  the  morning  had  by  the 
afternoon  gathered  itself  into  a  well-defined  secondary 
over  the  north  of  France.  At  the  same  time,  partly  by 
&  slight  general  upward  surge  or  increase  of  pressure — for 
the  lowest  isobar  in  the  second  chart  is  29*8  instead  of 
29 '7 — and  partly  by  the  advance  of  the  isobar  29*9  nearer 
to  London,  the  pressure  rose  in  that  city,  as  shown  in  the 
barogram,  while  the  rain  was  due  to  the  formation  of 
the  secondary.  In  this,  as  in  all  other  similar  instances, 
the  advance  of  pressure  from  the  north-west  appears  to 
develop  small  secondaries,  just  as  a  big  advancing  wave 
makes  small  eddies  in  front  of  itself. 

These  secondaries  give  rain  with  a  rising  barometer 
east  wind. 


BAIN  WITH  RISING  BAROMETER  AND  WEST  WIND. 

In  the  ordinary  course  of  depressions  the  barometer 
falls  before  rain,  because  the  centre  of  the  cyclone  con- 
tains the  rain-area;  and  if  all  cyclones  moved  along 
a  pretty  regular  path,  and  did  not  alter  much  either  in 
depth  or  extent,  then  we  should  never  fail  to  forecast  rain 
correctly  whenever  we  saw  the  mercury  began  to  fall,  and 
iine  weather  soon  after  the  barometer  began  to  rise.  But 


404  WEATHER. 

sometimes,  before  either  the  centre  or  trough  of  a  cyclone- 
has  reached  a  station,  the  depression  begins  to  fill  up  so 
rapidly  that  the  barometer  actually  rises,  though  in  front 
of  a  cyclone.  Then  we  get  rain  with  a  rising  barometer, 
but  the  sky  retains  the  appearance  due  to  the  front  of 
a  cyclone.  These  are  among  the  cases  when  synoptic 
charts  enable  us  to  explain  what  would  be  hopeless  with- 
out them,  and  to  see  the  truth  of  the  statement  that 
weather  depends  on  the  position  of  the  observer  in  a 
cyclone,  and  not  on  the  height  or  motion  of  his  barometer. 
The  next  example  will  illustrate  this  point  very  clearlyr 
and  also  the  complications  which  arise  when  a  secondary 
forms  in  rear  of  a  cyclone. 

The  following  sequence  of  weather  was  observed  by 
the  author  at  the  beginning  of  September,  1883,  about 
sixteen  miles  west  of  the  town  of  Leeds,  and  520  feet 
above  the  sea-level.  His  journal,  as  written  at  the  time, 
runs  thus : — 

"September  1,  1883. — Early,  blue  sky,  misty,  heavy 
dew,  wind  south ;  by  noon,  sky  threatening,  halo  of  46° 
diameter,  visible  from  12  to  2  p.m.,  then  overcast.  At 
4.45  p.m.  light  rain  began;  wind  to  south-east,  almost 
calm.  By  8  p.m.  rain  heavy ;  wind  up  and  more  to  east 
Barometer  fell  fast  all  day.  Kemark :  Very  slow 
coming  on. 

"  September  2. — Early,  a  gale ;  8  a.m.,  uniform  nimbus^ 
wind  south-east,  moderate;  the  same  all  day,  rain  off 
and  on,  often  rather  heavy,  but  the  wind  falling  light 
towards  night.  At  8  a.m.  it  was  seen  that  the  barometer 
had  been  falling  all  night,  and  was  then  very  low.  The 
mercury  continued  to  fall  all  day  till  6  p.m.,  when  it 


FORECASTING  FOR  SOLITARY  OBSERVERS.  405 

turned  without  a  squall ;  though  about  6.45  there  was 
a  passing  shower.  After  this  there  was  not  the  look  of 
the  rear  of  a  cyclone.  Bemark :  Eain  with  a  south-east 
wind  lasts  long ;  twenty-eight  hours. 

"  September  3. — Warm,  wet,  and  stormy  ;  soft ;  south- 
south-west  to  south-west,  fresh  to  strong.  About  5.30  a.m. 
a  squall ;  6.30,  heavy  shower,  wind  round.  All  day  dirty, 
misty,  driving  showers,  though  barometer  rising  fast. 
About  5  p.m.  rain  off,  but  soft  stratus,  not  cumulus  of 
cyclone-rear;  night  overcast.  Kemarks:  Weather  like 
front,  not  rear  of  a  cyclone,  and  much  worse  than  the  two 
previous  days,  when  the  barometer  was  falling." 

By  next  day,  the  sky  was  bright,  and  covered  with 
cumulus  and  occasional  showers,  as  is  usual  in  rear  of  a 
cyclone,  while  the  wind  was  round  to  the  north-west. 
What  we  have,  then,  to  explain  is,  first,  twenty-eight 
hours'  rain  with  a  falling  barometer,  and  then  twenty- 
three  hours'  rain,  and  worse  weather  after  the  mercury 
began  to  rise  ;  also,  not  only  the  increased  severity  of  the 
weather,  but  why  the  sky  did  not  assume  its  ordinary 
appearance  after  the  centre  of  the  cyclone  had  apparently 


In  Figs.  88  and  89  we  give  the  6  p.m.  chart  for  Sep- 
tember 2,  and  also  that  for  8  a.m.  the  next  day,  on  a 
large  scale ;  the  coast  lines  are  omitted  for  the  sake  of 
clearness,  but  the  position  of  the  letters  w,  p,  D,  for 
Wick,  Penzance,  and  Dover,  will  sufficiently  indicate  the 
scale  of  the  chart.  The  spot  marked  L  shows  the  station 
where  the  observations  were  made. 

At  6  p.m.,  September  2nd,  the  centre  of  a  cyclone  of 
considerable  intensity  lay  near  Loughborough,  but  the 


406 


WEATHER, 


isobar  of  28*7  ins.  had  the  form  of  an  ellipse,  whose 
longer  axis  lay  from  about  Barrow  to  near  Cambridge. 
This  gives  the  direction  of  the  trough  of  the  cyclone. 


2-Q-Sj 

6.  p.  m. 


29.0 


.w 


FIG.  88. — Bad  weather  with  rising  barometer. 

which  in  this  case  forms  an  acute  angle  of  60°  with  the 
direction  of  the  path  of  the  cyclone. 

If  we  assume  that  the  cyclone  travelled  at  a  uniform 
rate  till  8  a.m.  next  morning,  it  must  have  been  at  least 


FOKECASTING  FOR  SOLITARY   OBSERVERS. 


407 


three  hours  before  the  trough  of  the  cyclone  passed  near 
Leeds,  that  is  to  say,  before  the  point  on  the  trough 
marked  0*67  in  Fig.  88  reached  L.  The  barogram  at  L, 


FIG.  89. — Bad  weather  with  rising  barometer. 

however,  began  to  rise  at  6  p.m.,  and  the  explanation  of 
this  apparent  anomaly  is  as  follows  : — 

At  6  pm.  on  the  2nd  (Fig.  88),  the  lowest  barometer 
marked    28*66    ins.,   while    by    8    a.m.    next    morning 


408  WEATHEE. 

(Fig.  89)  the  lowest  pressure  was  only  about  29'0  ins., 
so  that  the  cyclone  had  filled  up  by  0*34  inch  during 
those  fourteen  hours,  or  at  the  rate  of  0*024  inch  per  hour. 

It  would,  therefore,  appear  that  the  barometer  rose  for 
at  least  three  hours  near  Leeds,  while  the  centre  of  the 
cyclone  was  still  approaching,  because  the  rise  of  the 
mercury  due  to  the  filling  up  of  the  cyclone  was  greater 
than  the  fall  of  the  barometer  owing  to  the  approach  of 
the  cyclone- trough. 

The  actual  figures  in  this  instance  were  as  follows : — 
If  there  had  been  no  filling  up,  the  mercury  should  have 
fallen  0*03  inch  during  the  three  hours  6  to  9  p.m.  This 
is  got  at  as  follows  : — The  difference  of  pressure  between 
L,  28*7  ins.,  and  the  point  on  the  trough  marked  0*67 
is  0*03  inch,  and  the  distance  between  the  two  points  is 
fifty  miles. 

This  would  be  traversed  in  three  hours,  because  the 
cyclone-centre  moved  two  hundred  and  thirty  miles  to 
the  point  marked  8  a.m.,  3rd  (Fig.  88),  in  the  fourteen 
hours  which  elapsed  between  the  times  for  which  the 
charts  are  constructed. 

If  there  had  been  no  advance,  but  only  a  filling  up  of 
the  cyclone,  the  mercury  would  have  risen  0'07  inch  per 
hour.  Therefore  the  balance  of  rise  over  fall  should  have 
been  0*04  inch,  and  this  was  the  amount  actually  ob- 
served. At  8  a.m.,  September  3rd  (Fig.  89),  we  see  that 
the  centre  of  the  primary  cyclone  was  north-north-east  of 
Leeds,  and  about  two  hundred  miles  distant.  The  centre 
was  also  moving  north,  so  that  the  motion  of  the  barometer 
would  be  upwards  from  the  action  of  the  primary.  There 
is,  however,  a  marked  irregularity  in  the  lie  of  the  isobars 


FORECASTING  FOR  SOLITARY   OBSERVERS.  40(J 

over  the  Irish  Channel,  which  points  to  the  existence  of 
a  secondary  in  that  neighbourhood.  The  chart  at  6  p.m. 
the  same  day  showed  that  the  secondary  which  lay  over 
the  Irish  Channel  at  8  a.m.  had  become  more  pronounced, 
and  had  then  its  lowest  portion  near  Liverpool.  Con- 
sequently Leeds  and  its  neighbourhood  were  still  under 
the  influence  of  the  front  of  this  secondary,  though  the 
mercury  had  risen  about  O2  inch,  partly  owing  to  the 
progress  of  the  primary,  and  partly  also  to  the  cyclone 
gradually  filling  up. 

By  next  day  the  charts  showed  that  the  primary  had 
moved  still  further  to  the  north,  and  that  the  secondary 
was  lying  over  the  North  Sea,  so  as  to  form  a  sort  of  V-- 
shaped depression  to  the  south  of  the  primary  cyclone. 

The  explanation  of  the  apparently  anomalous  weather 
is  then  very  simple.  The  first  twenty-eight  hours  of  rain 
with  a  falling  barometer  were  due  to  the  front  of  a 
primary  cyclone  which  was  moving  very  slowly ;  and  so 
far  this  represents  the  usual  sequence  of  weather  in  such 
cases.  The  first  three  of  the  twenty-three  hours  of  rain 
with  worse  weather  after  the  barometer  began  to  rise 
were  also  due  to  the  cyclone-front,  though  the  mercury 
rose  from  filling  up.  The  remaining  twenty  hours  of 
rain  and  the  characteristic  sky  of  the  front  of  a  cyclone 
were  due  to  the  formation  of  a  secondary  in  rear  of  the 
primary ;  so  that  though  the  barometer  was  rising,  owing 
to  the  passage  and  filling  up  of  the  primary,  still  Leeds 
was  during  the  whole  of  that  day  exposed  to  the  influence 
of  the  front  of  the  secondary  with  its  characteristic  dirty 
weather.  The  wind  was  stronger  after  the  glass  began 
to  rise,  because  the  gradients  were  steeper  in  rear  of  the 


410  WEATHER. 

primary  than  they  had  been  in  most  portions  of  its  front. 
By  the  fourth  day  the  secondary  had  passed  away,  and 
then  the  typical  weather  of  the  rear  of  a  cyclone  was 
experienced. 

KAIN  WITH  STEADY  BAROMETER. 

So  far  for  rain  with  a  rising  barometer;  now  we 
must  consider  precipitation  with  a  steady  barometer.  To 
Englishmen  this  is  more  perplexing  than  rain  with  a 
rising  mercury. 

In  the  latter  case,  we  see  at  once  that  there  is  some 
disturbance  going  on ;  but  in  the  former  we  often  have  a 
steady  downpour  for  several  hours,  with  an  absolutely 
steady  barometer.  Kain  of  this  class  is  much  more 
common  in  continental  Europe  than  in  Great  Britain, 
except  in  one  very  rare  case,  which  will  be  mentioned 
hereafter. 

The  rain  is  always  either  non-isobaric,  or  of  that  kind 
which  is  associated  with  secondaries  and  not  with  primary 
cyclones.  For  this  reason,  the  rain  is  never  accompanied 
by  a  gale  of  wind,  though  there  are  often  angry  gusts  at 
the  beginning  and  end  of  the  rainfall. 

In  Fig.  90  we  give  a  photographic  engraving  of  the 
author's  barographic  trace  in  London,  on  July  1  and  2, 
1877.  This,  being  absolutely  untouched  by  hand,  gives 
the  minute  irregularities  of  pressure  in  a  manner  which 
no  hand-copied  diagram  can  ever  do.  The  horizontal 
lines  represent  differences  of  half  an  inch  of  pressure,  the 
thickest  one  marking  the  level  of  29*5  inches.  The 
horizontal  lines  are  drawn  at  six-hour  intervals. 


FORECASTING  FOR  SOLITARY  OBSERVERS.     411 

At  first  sight,  there  might  seem  to  be  little  sign  of 
any  disturbance,  for  the  actual 
changes  of  barometric  level  are 
insignificant,  and  the  diurnal 
variation  is  more  obvious  than 
usual  in  Great  Britain.  If, 
however,  we  look  carefully  at 
the  trace,  we  shall  find  that  just 
before  6  a.m.  on  July  1  there  is  a 
very  small  dip  of  the  barometer, 
and  that  then  the  trace  is  al- 
most quite  straight  till  about 
4.30  p.m.,  when  there  is  another 
small  dip;  after  which  the 
regular  diurnal  variation  is 
absolutely  undisturbed.  In 
London  rain  commenced  at  the 
first  dip,  and  continued  without 
intermission  till  the  second, 
after  which  the  sky  cleared. 

The  charts  for  that  day, 
which  unfortunately  the  num- 
ber of  illustrations  at  our  dis- 
posal does  not  admit  of  repro- 
ducing, show  that  this  was  all 
caused  by  the  formation  and 
passage  of  a  small  secondary 
over  the  north  of  France  and 
the  English  Channel ;  and  both 
the  rain  and  the  barographic  trace  are  most  characteristic 
of  this  class  of  depression.  A  case  of  this  sort  shows, 


412  WEATHER. 

more  than  any  other,  the  superior  value  of  a  continuous 
trace  over  an  intermittent  barograph ;  for,  though  the 
latter  permits  of  the  tabulation  of  hourly  values  for 
the  determination  of  diurnal  variations,  they  entirely  lose 
all  chance  of  following  the  more  minute  alterations  of 
pressure,  which  are  often  accompanied  by  great  changes 
of  weather.  The  most  interesting  point  about  secondaries 
is  the  contrast  between  the  intensity  of  the  weather  which 
they  induce  and  the  apparently  small  disturbance  of 
pressure.  In  primary  cyclones  the  gradients  are  to  a 
€ertain  extent  a  good  measure  of  the  intensity.  In 
secondaries,  on  the  contrary,  the  rainfall  has  no  relation 
whatever  to  the  barometric  disturbance.  This,  of  course, 
makes  it  very  difficult  for  the  forecaster.  All  he  can  say 
when  he  sees  a  secondary  is — rain ;  but  he  can  give  no 
estimate  of  the  quantity  of  precipitation,  as  he  can  of  the 
force  of  the  wind  in  a  primary  cyclone. 

Earely  in  Great  Britain,  frequently  in  continental 
Europe,  habitually  in  the  tropics,  we  have  purely  non- 
isobaric  rains,  totally  unconnected  with  any  secondary. 
These  are  often  indicated  on  the  barographic  trace  by  a 
sudden  sharp  rise  of  the  type  we  illustrated  in  our  chapter 
on  Thunderstorms.  This  is  probably  a  purely  local  effect 
of  a  heavy  downpour  pressing  the  air  down  by  its  own 
weight. 

The  other  case  of  rain — this  kind  often  with  a  gale  of 
wind — with  an  apparently  steady  barometer,  only  occurs 
in  very  unsettled  weather.  In  our  chapter  on  Weather- 
Types,  we  gave  several  examples  of  nearly  stationary 
cyclones,  which  increased  much  in  depth,  while  some  of 
the  adjacent  anticyclones  increased  in  height.  As  a 


FORECASTING  FOR  SOLITARY  OBSERVERS. 

necessary  consequence,  there  must  be  some  station  where 
no  change  of  pressure  would  be  observed ;  but  on  one 
side  pressure  would  decrease,  while  it  increased  on  the 
other;  so  by  this  means  very  steep  gradients  might  come 
to  lie  over  the  station.  The  wind  would  rise  to  a  gale, 
while  the  weather  would  conform  to  the  shape  of  the 
isobars,  but  the  mercury  would  remain  stationary;  we 
might,  in  fact,  say  that  the  station  was  "  nodal "  as 
regards  the  fluctuations  of  surrounding  pressure. 

It  would  be  an  extreme  case  when  no  change  of 
pressure  took  place,  and  could  only  happen  at  a  limited 
number  of  places.  But  under  the  same  conditions  there 
will  always  be  a  number  of  stations  where  only  a  moderate 
fall  of  the  barometer  takes  place,  but  a  gale  out  of  all  pro- 
portion to  the  apparent  depression  is  experienced.  This 
illustrates  the  important  difference  between  the  fall  of 
the  barometer  clue  to  the  passage  of  a  well-defined 
cyclone,  and  that  due  to  the  rearrangement  of  the  distri- 
bution of  pressure  round  the  station.  As  an  example,  we 
may  turn  to  Fig.  93  in  the  next  chapter,  where  we  give 
two  charts  of  North- Western  Europe,  on  February  6, 
1883,  at  8  a.m.  and  6  p.m.  respectively.  The  position  of 
the  isobar  of  30'4  ins.  is  practically  the  same  in  both 
maps ;  but  between  the  morning  and  evening  observations* 
pressure  has  fallen  O4  inch  in  the  west  of  Ireland,  and 
risen  0*2  inch  over  Sweden.  The  shape  of  the  isobars  has 
not  altered  much,  so  that  gradients  have  become  steep, 
with  little  change  of  wind-direction.  Thus  many  stations, 
near  the  nodal  isobar,  will  experience  an  increase  of  wind 
with  either  a  rising,  stationary,  or  slightly  falling  barometer. 
For  instance,  at  Aberdeen,  marked  A,  the  wind-arrow 


414  WEATHER. 

shows  that  the  wind  had  risen  from  a  fresh  breeze  to  a 
moderate  gale ;  while  the  motion  of  the  isobars  does  not 
indicate  a  fall  of  more  than  Ol  inch  in  the  ten  hours 
which  elapsed  between  the  two  sets  of  observations. 

FINE  WEATHER  WITH  Low  OR  FALLING  BAROMETER. 

From  the  above,  in  which  the  weather  is  out  of  all 
proportion  to  the  depression  of  the  mercury,  we  readily 
pass  to  the  converse  case,  in  which  the  fall  of  the 
barometer  is  quite  disproportioned  to  the  severity  of  the 
weather  which  is  afterwards  experienced.  In  the  North 
of  Europe,  during  the  winter  months,  and  when  the 
westerly  type  of  weather  prevails,  the  barometer  will 
sometimes  fall  half  an  inch  or  more,  and  often  below 
28*5  ins.,  while  no  strong  winds  follow,  and  the  general 
appearance  of  the  sky  is  bright,  with  perhaps  a  little 
cumulus  cloud.  This  also  is  readily  explained  by  reference 
to  our  large  Atlantic  charts.  In  them  we  saw  that  when 
the  Atlantic  is  covered  by  a  persistent  area  of  low  pressure, 
the  depth  of  the  lowest  point  often  suddenly  decreases 
nearly  an  inch,  and  that  the  gradients  near  the  centre 
are  very  slight.  In  some  phases  of  that  type  of  weather, 
the  area  of  low  pressure  stretches  over  Europe,  and  the 
minimum  of  this  area  rises  up  and  down  exactly  as  when 
the  centre  lies  over  the  ocean. 

If,  then,  Great  Britain,  for  instance,  lay  within  that 
area,  pressure  might  decrease  a  whole  inch,  and  neither 
storm  nor  rain  be  experienced.  The  great  fall  of  pressure 
would,  of  course,  develop  steep  gradients,  somewhere  to 
the  west  of  those  islands  ;  but  as  the  depression  was  not 


FORECASTING  FOR  SOLITARY   OBSERVERS.  415 

caused  by  the  drifting  past  of  a  cyclone,  neither  wind  nor 
rain  would  follow  in  England.  The  centre  of  these  great 
depressions,  which  are  not  true  cyclones,  is  usually  asso- 
ciated with  cool,  bright  weather  and  cumulus  cloud,  and 
therefore  weather  of  that  description  would  probably  be 
experienced.  From  a  case  of  this  sort,  we  learn  how  to 
avoid  the  popular  errors  that  the  violence  of  a  gale  is 
always  proportional  to  the  fall  of  the  barometer,  and  that 
a  very  low  barometer  is  necessarily  associated  with  very 
bad  weather. 


COMPLICATIONS  ON  BOARD  SHIP. 

All  the  examples  which  we  have  now  given  in  this 
•chapter  will  sufficiently  explain  the  nature  of  forecasting 
by  means  of  a  single  barometer  and  observations  on  the 
appearance  of  the  sky,  as  also  the  true  nature  of  the 
apparent  exceptions  to  the  ordinary  relationship  between 
weather  and  the  movements  of  the  mercury  in  a  barometer 
tube. 

Our  space,  unfortunately,  does  not  permit  us  .  to 
describe  the  still  greater  difficulties  which  occur  when 
the  observations  are  taken  on  board  a  moving  ship ;  then, 
of  course,  we  have  not  only  the  motion  of  cyclones,  but 
also  that  of  the  ship  to  take  into  account,  and  it  is 
manifest  that  many  of  the  rules  which  we  have  laid  down 
for  land-stations  would  require  considerable  modification. 
The  same  limitation  of  space  also  compels  us  to  omit  the 
notice  of  the  theory  of  handling  ships  in  the  small  cyclones 
which  occur  in  tropical  countries  under  the  names  of 
hurricanes,  typhoons,  etc.,  but  the  author  hopes  to  make 
this  branch  of  meteorology  the  subject  of  another  work. 


416  WEATHER. 


CHAPTER  XV. 

FORECASTING  BY  SYNOPTIC  CHARTS. 
STATEMENT  OF  THE  PROBLEM. 

BY  synoptic  forecasting  we  mean  that  branch  of  weather- 
prevision  which  is  carried  on  by  means  of  synoptic  charts. 
The  forecaster  in  a  central  bureau  is  in  telegraphic  com- 
munication with  observers  for  many  hundred  miles  round. 
From  their  reports  he  constructs  synoptic  charts  at  such 
intervals  as  seem  necessary.  To  the  indications  which 
he  derives  from  the  appearance  of  these  maps,  he  adds 
all  his  own  accumulated  experience  of  the  nature  of  the 
meteorology,  and  the  motion  of  depressions  in  his  own 
country  ;  and  also  such  knowledge  of  the  recurrent 
periods  of  different  kinds  of  weather  as  he  may  be 
acquainted  with.  From  all  that  he  forms  his  own 
judgment  as  to  what  changes  are  likely  to  take  place,  and 
issues  his  forecast  accordingly. 

From  the  nature  of  things  there  can  never  be  many 
forecasters.  The  rapid  nature  of  meteorological  changes 
makes  the  employment  of  the  electric  telegraph  absolutely 
necessary,  and  the  great  expense  which  is  thereby  in- 


FORECASTING  BY  SYNOPTIC  CHARTS.  417 

curred,  compared  with  the  uncommercial  nature  of  the 
results,  practically  relegates  forecasting  to  the  functions 
of  a  Government  office. 

From  the  preceding  chapters  we  now  know  what 
weather  is.  Instead  of  dealing  with  abstractions  called 
wind,  rain,  cloud,  heat,  etc.,  we  have  gradually  been  led 
up  to  the  idea  that  all  meteorological  phenomena  are 
the  products  of  the  motion  and  circulation  of  a  moist 
atmosphere.  Now  we  know  that  when  we  talk  about 
forecasting  weather,  we  mean  that  we  are  going  to  say 
how  or  where  certain  aerial  eddies  will  move,  or  when 
new  ones  are  likely  to  form;  also  whether  any  cyclone 
will  be  violent  or  gentle. 


AIDS  TO  FORECASTING. 

In  this  chapter  we  propose  to  make  some  additional 
remarks  on  the  whole  aspect  of  the  subject.  We  shall 
enumerate  several  aids  to  forecasting  which  can  be 
obtained  from  various  sources,  and  point  out  both  the 
present  difficulties  and  the  future  possibilities  of  weather- 
prevision.  Finally,  we  shall  give  some  examples  of 
successful  and  unsuccessful  forecasts  in  different  countries, 
and  an  account  of  the  various  percentages  of  success 
which  the  different  offices  have  achieved.  In  an  inter- 
national work  we  shall  better  illustrate  the  general 
principles  of  the  subject  by  exemplifying  forecasts  in 
different  countries  than  by  trying  to  give  any  one  in 
detail.  A  tolerably  full  account  of  the  nature  of  forecast- 
ing, and  of  the  details  of  the  methods  and  machinery  for 
issuing  storm-warnings  in  Great  Britain,  will  be  found  in 

2  E 


418  WEATHER. 

the  author's  work,  "  Principles  of  Forecasting  by  Means 
of  Weather  Charts,"  issued  by  the  authority  of  the 
Council  of  the  Meteorological  Office. 

UNEQUAL  BAROMETRIC  CHANGES. 

We  have  already  fully  explained  the  use  of  the  recog- 
nition of  weather-types  in  every  country,  during  which 
sequence  the  motion  of  depressions  follow  either  a  certain 
general  direction  or  maintain  a  certain  general  position  ; 
but  in  variable  climates  we  often  find  tracts  of  weather 
which  can  be  assigned  to  no  particular  type.  The  fore- 
caster is  then  at  a  great  disadvantage,  for  he  has  little  to 
guide  him  a&  to  the  future. 

The  very  idea  of  weather-type  involves  the  knowledge 
that  the  sequence  of  changes  will  follow  in  a  certain 
groove,  so  that  when  no  type  is  obvious,  there  is  little 
basis  on  which  to  frame  a  forecast.  In  most  cases  the 
forecaster  has  to  rely  on  the  difference  of  barometric  rate 
in  various  districts.  If  he  sees  that  the  barometer  is 
falling  much  more  rapidly  in  one  district  than  in  any 
other — even  if  no  definite  depression  is  formed — he  knows 
that  steeper  gradients  must  thereby  be  formed,  so  that 
the  wind  must  increase,  and  whatever  weather  is  due  to 
the  existing  shape  of  isobars  will  get  worse. 

Conversely,  if  he  finds  pressure  increasing  in  a  district 
of  low  barometer,  he  knows  that  gradients  will  decrease, 
and  that  both  wind  and  weather  will  moderate.  The 
details  vary  indefinitely,  and  no  rule  can  be  laid  down 
even  for  a  single  country ;  everything  must  be  left  to  the 
judgment  and  experience  of  the  forecaster. 


FORECASTING  BY  SYNOPTIC   CHARTS.  419 


CYCLONE-PATHS. 

The  paths  of  cyclones,  and  the  nature  of  the  influences 
which  deflect  or  otherwise  alter  them,  are  so  important 
that  we  propose  to  devote  some  paragraphs  to  their  con- 
sideration, of  course  with  a  special  reference  to  the  bear- 
ing which  they  have  on  forecasting. 

When  the  paths  of  the  rare  but  violent  cyclones  of 
the  tropics,  which  are  known  as  hurricanes,  typhoons,  or 
cyclones,  are  plotted  on  a  chart,  we  find  that,  though 
there  is  a  general  similarity  in  their  tracks,  there  is  still 
so  much  difference  that  we  cannot  attempt  to  lay  down 
any  absolute  law  of  their  motion. 

For  instance,  the  West  India  hurricanes  usually 
begin  with  a  westward  course,  and  then  gradually  bend 
round  till  they  end  by  moving  towards  the  east  or  north- 
east. But  in  some  instances  they  continue  in  a  westerly 
direction,  and  traverse  the  southern  portion  of  the 
American  Union,  instead  of  curving  round  across  the 
Atlantic. 

For  this  reason,  if  a  ship  was  handled  on  the  supposition 
that  the  hurricane  would  always  go  the  same  course,  she 
would  be  exposed  to  very  great  danger. 

In  the  temperate  zone,  where  cyclone-paths  are  still 
more  irregular,  any  attempt  to  lay  down  any  hard  and 
fast  rule  for  the  tracks  of  depressions  could  only  lead  to 
disastrous  failure  of  any  forecasts  which  were  based  on 
that  system;  but  though  the  numerous  causes  which 
have  been  found  to  modify  the  paths  of  cyclones  cannot 
be  allowed  for  in  estimating  the  probable  future  path  of 


420  WEATHER. 

any  actual  depression,  still  many  points,  which  have  been 
noted,  are  so  interesting  that  we  shall  mention  some  of 
them  more  in  detail. 


TENDENCY  TO  FOLLOW  CEKTAIN  TRACKS. 

During  the  persistence  of  any  type,  two  or  three 
successive  cyclones  have  a  remarkable  tendency  to  follow 
the  same  course.  This,  of  course,  is  the  natural  product 
of  the  fact  that  the  path  of  a  cyclone  is  determined  by 
the  type  of  pressure  in  which  it  is  formed.  Sometimes 
this  path  is  entirely  dictated  by  surrounding  pressure ; 
but  at  other  times  local  configuration  of  the  land  exercises 
a  most  powerful  directive  influence. 

For  instance,  in  Great  Britain,  during  the  westerly 
type,  when  the  depressions  are  so  far  south  as  to  cross 
that  island,  the  centres  have  a  decided  tendency  to 
traverse  either  the  line  of  the  Caledonian  Canal  in  Scotland, 
or  the  low-lying  ground  which  separates  the  valleys  of 
the  Forth  and  Clyde.  Both  of  these  courses  coincide 
with  what  we  may  call  lines  of  least  resistance,  for  these 
are  the  two  easiest  lines  by  which  it  is  possible  to  cross 
the  mountainous  districts  of  Scotland.  Another  well- 
marked  tendency  of  cyclone-centres  is  to  hug  the  sea- 
shore, rather  than  to  strike  inland.  When  a  cyclone 
comes  up  the  English  Channel,  it  often  skirts  the  south 
coast  of  England,  and  then  moves  more  northward  along 
the  east  coast,  rather  than  pass  directly  to  the  north-east 
across  the  land.  In  like  manner,  large  cyclones  which 
come  in  from  the  Atlantic,  when  they  meet  the  coast  of 
Norway,  often  hug  the  coast  for  several  days,  instead  of 


FORECASTING  BY   SYNOPTIC   CHARTS.  421 

going  straight  to  the  north-east.  In  the  United  States 
the  great  majority  of  cyclones  traverse  the  line  of  the 
great  lakes,  and  then  either  follow  the  valley  of  the  St. 
Lawrence  or  strike  across  the  New  England  States  into 
the  Atlantic. 

Great  chains  of  mountains  also  influence  very  power- 
fully the  paths  of  cyclones. 

In  Europe,  the  chain  of  the  Alps  almost  forms  a  natural 
boundary  between  the  weather  of  the  Mediterranean  and 
that  of  the  northern  portion  of  the  continent.  As  a  rule, 
that  great  inland  sea  has  a  totally  different  atmospheric 
circulation  from  that  which  affects  the  rest  of  Europe. 

This  will  be  very  obvious  if  we  turn  again  to  the  large 
charts  which  we  gave  in  our  chapter  on  Weather-Types. 

Sometimes  we  can  trace  a  cyclone  in  the  Mediterranean 
trying  to  cross  the  Alps,  and  being  broken  up  in  the 
attempt.  We  can  readily  understand  that  if  a  mountain 
chain,  12,000  feet  high,  sliced  off  the  lower  half  of  such 
a  shallow  and  complex  vortex  as  a  cyclone,  the  whole 
system  might  very  easily  be  destroyed.  Exceptional 
cases,  however,  do  occur  in  which  large  cyclones  cross 
the  great  barrier  of  the  Alps. 

In  India,  too,  the  still  loftier  chain  of  the  Himalayas 
imposes  an  even  greater  influence  on  the  meteorology  of 
that  country,  as  a  glance  at  the  charts  which  we  have 
already  given  of  the  monsoon  districts  will  abundantly 
show. 

STORMS  CROSSING  THE  ATLANTIC. 

But  the  cyclones  whose  motions  have  created  by  far 
the  greatest  interest  in  Europe  are  those  which  sometimes 


422  WEATHER. 

come  across  the  Atlantic.  The  public  have  been  fasci- 
nated by  the  idea  that  a  storm  could  be  telegraphed  from 
New  York,  and  its  arrival  on  the  coasts  of  Europe  foretold 
three  or  four  days  in  advance.  If  cyclones  only  moved 
with  tolerably  uniform  velocities  and  in  tolerable  uniform 
paths,  and  the  intensity  remained  constant,  then,  indeed, 
it  would  often  be  possible  to  obtain  timely  warning  from 
the  United  States  or  Canada.  Although  the  diagrams 
which  we  have  already  given  of  Atlantic  weather  would 
sufficiently  show  the  real  character  of  Atlantic  cyclones, 
still  the  nature  of  the  paths  of  these  depressions  will  be 
more  clearly  understood  if  we  give  the  tracks  of  all  the 
depressions  which  appeared  in  the  Atlantic  during  a 
single  month.  This  will  do  as  a  sample  of  any  other 
month  or  season.  In  Fig.  91  we  therefore  give  a  chart 
of  all  the  cyclones  which  could  be  traced  for  more  than 
two  days  in  the  United  States,  the  Atlantic,  and  Europe 
during  the  month  of  July,  1879.  During  that  month 
there  were  seven  well-defined  cyclone-tracks  within  the 
above-mentioned  area. 

These  paths  are  plotted  on  our  chart,  and  the 
position  of  the  centre  of  each  cyclone  on  every  day  is 
clearly  marked. 

Now,  the  first  glance  will  at  once  satisfy  us  as  to  the 
broad  idea  that  cyclones  usually  move  in  a  certain 
general  direction. 

The  whole  of  the  paths  lie  along  a  comparatively 
narrow  belt  of  the  ocean  ;  but  when  we  come  to  look  into 
the  details,  we  shall  find  that  the  smaller  variations  of 
motion  effectually  preclude  the  use  of  this  knowledge  in 
forecasting. 


FORECASTING  BY   SYNOPTIC  CHARTS. 


423 


Of  the  seven  cyclones,  four — Nos.  I.,  II.,  V.,  and  VII. 
— were  formed  in  mid-Atlantic,  and  then  pursued  a  more 
or  less  irregular  course  towards  Europe.  Observe  how 
the  curious  loop  to  the  northwards,  which  the  path  of 
No.  I.  makes  at  the  beginning  of  the  month,  is  almost 
exactly  reproduced  at  the  end  of  that  time  by  cyclone 


FIG.  91. — Cyclones  crossing  the  Atlantic. 

No.  III.  Cyclones  IV.  and  VI.  were  formed  over  the 
United  States ;  both  passed  into  the  Atlantic,  but  neither 
reached  the  coasts  of  Europe. 

Cyclone  No.  III.  also  had  its  origin  in  the  American 
Union,  though,  unlike  the  two  others,  it  not  only  survived  its 
journey  across  the  Atlantic,  but,  after  traversing  Europe, 


424  WEATHER. 

passed  into  Siberia.  Our  chart  follows  its  history  for  the 
ten  days  from  July  9  to  28 ;  but  let  us  try  to  see  how  we 
should  fare  if  we  attempted  to  issue  forecasts  on  the 
supposition  that  the  depression  would  move  either  in  a 
uniform  direction  or  with  a  uniform  velocity.  From  the 
9th  to  the  llth  the  cyclone  moved  towards  the  north- 
east with  a  considerable  velocity ;  the  next  two  days  it 
turned  to  the  south-east  with  diminished  speed,  and  left 
the  shores  of  the  United  States  with  a  south-easterly 
trajectory.  The  day  of  leaving  the  velocity  increased; 
but  by  next  morning  the  direction  changed  again  to  the 
north-east,  and  the  velocity  gradually  diminished  for  the 
next  seven  days,  by  which  time  the  depression  had 
reached  the  coast  of  Ireland,  after  being  eight  days  in 
transit  from  Nova  Scotia.  A  crack  steamer  would  have 
done  the  distance  in  five  days.  From  that  day,  the  21st, 
the  speed  increased  again,  and  the  cyclone  turned  still 
more  towards  the  north.  Then,  with  gradually  decreasing 
velocity,  the  path  bent  round  to  the  south,  and  afterwards 
turned  once  more  to  the  northwards,  with  increased 
speed,  till  the  28th  of  July,  when  we  lose  sight  of  the 
depression  in  the  frozen  marshes  of  Siberia. 

This  example  will  abundantly  prove  that  we  can  form 
no  estimate  of  the  future  path  or  velocity  of  a  cyclone- 
centre  by  any  observations  on  its  earlier  motion.  In  this 
case  the  direction  and  velocity  of  the  depression  when  it 
left  the  American  shore  gave  no  clue  either  to  its  path 
across  the  ocean,  or  its  meanderings  after  reaching  the 
continent  of  Europe. 

There  is  another  point  which  we  must  remember  in 
the  discussion  of  this  question — we  track  cyclones,  but 


Of    TH£ 

TJNJVEK8ITY  )] 

CAUI 

FORECASTING   BY   SYNOPTIC   CHARTS.  425 

not  necessarily  storms.  The  size  and  intensity  of  this 
cyclone  varied  every  day  of  its  life.  Some  days  the 
intensity  was  so  great  that  the  wind  rose  to  the  force 
of  a  gale  in  places  ;  other  days  the  gradients  were  never 
developed  of  sufficient  steepness  to  give  rise  to  more 
than  a  breeze.  No  general  rule  can  be  laid  down  that 
will  apply  to  the  life-history  of  a  cyclone ;  we  must 
watch  from  day  to  clay  for  symptoms  of  increasing  or 
decreasing  intensity. 

From  all  this  we  can  also  estimate  the  value  of  the 
idea  that  a  swift  Atlantic  mail-steamer  could  arrive 
before  a  storm,  and  so  give  notice  of  approaching  danger. 

The  cyclone  which  we  have  just  traced  travelled  rather 
slower  than  usual;  we  often  find  depressions  cross  the 
Atlantic  in  four  days.  However,  in  this  case,  the  cyclone 
came  across  at  just  about  the  speed  of  the  fastest  steamers. 
The  first  two  days  the  cyclone  would  have  been  passing 
the  vessel ;  on  all  the  other  six  days,  the  steamer  would 
have  been  catching  up  the  cyclone.  The  ocean  route 
from  the  mouth  of  the  St.  Lawrence  to  Cork  is  almost 
exactly  along  the  track  of  tins  cyclone.  A  steamer 
would,  therefore,  have  experienced  little  wind,  but  a 
uniformly  low  barometer  during  her  voyage.  Any  report 
which  she  alone  could  give  would  be  useless  to  a  fore- 
caster in  London  or  Paris;  but  if  several  boats  were 
arriving,  and  they  all  telegraphed  up  their  observations 
at  8  a.m.  on  the  three  or  four  preceding  days,  then  the 
combination  of  their  results  would  certainly  enable  the 
forecaster  to  deduce  some  useful  indications. 

In  all  British  forecasting  a  certain  amount  of  un- 
certainty must  always  remain  as  to  the  future  path  of  a 


426  WEATHER. 

cyclone,  even  when  we  see  a  well-defined  depression 
lying  off  the  coasts  of  Ireland ;  how  much  greater  must 
the  uncertainty  be  when  we  attempt  to  forecast  the  path 
of  a  cyclone  four  days  ahead,  and  from  a  distance  of  three 
thousand  miles?  If  the  forecaster  cannot  hit  England 
straight  when  he  aims  from  Ireland,  will  he  be  likely  to 
hit  her  at  all  if  he  shoots  from  New  York  ?  The  number 
of  cyclones  which  actually  cross  the  Atlantic  from  shore 
to  shore  appears  to  vary  from  about  eight  to  twenty  in  any 
year.  In  many  cases  it  is  difficult  to  say  whether  it  is 
the  same  cyclone  which  we  trace,  from  the  peculiar  manner 
in  which  two  depressions  may  fuse  into  a  single  new  one. 
On  the  whole,  then,  we  see  that  the  crude  notion  of 
forecasting  European  storms  from  the  United  States 
contains  some  elements  of  truth,  but  that  still,  from  the 
nature  of  cyclone-motion,  the  idea  can  never  be  used  in 
practical  forecasting. 

PATH  AS  INDICATED  BY  THE  STRONGEST  WIND  AND 
HIGHEST  ADJACENT  PRESSURE. 

A  good  deal  of  work  has  been  done,  both  in  England 
and  Germany,  on  the  question  of  how  far  the  path  of  a 
cyclone  can  be  determined  by  the  general  direction  or 
force  of  the  surrounding  wind,  and  the  investigators  have 
found  that  generally  the  propagation  of  the  cyclone  is  in 
the  same  direction  as  the  strongest  surface-wind  in  the 
neighbourhood.  There  are,  of  course,  a  good  many 
exceptions ;  and  it  is  impossible  in  our  present  state  of 
knowledge  to  say  whether  the  strongest  wind  indicates 
the  general  direction  of  the  generating  current  in  which 


FORECASTING  BY   SYNOPTIC   CHARTS.  427 

the  cyclone  is  only  an  eddy,  or  whether  the  strongest 
wind  is  the  product  of  the  combination  of  surface  rotation 
and  propagation,  being  nearly  in  the  same  direction  at 
one  particular  point. 

All  our  charts  have  shown  that  a  cyclone  usually  tries 
to  keep  an  area  of  high  pressure  on  its  right-hand  side ; 
and  this,  too,  has  a  good  deal  to  do  with  the  strongest 
wind  being  found  at  right  angles  to  the  centre,  and 
therefore  nearly  in  the  same  direction  as  the  motion  of 
the  whole  depression. 

INFLUENCE  OF  SURROUNDING  TEMPERATURE. 

We  now  come  to  the  far  more  difficult  but  important 
question  as  to  the  influence  of  surrounding  temperature 
on  the  propagation  of  cyclones,  and  as  to  whether  the 
development  of  heat  on  the  right-hand  side  of  a  cyclone 
is  the  cause  or  product  of  cyclone-motion. 

Putting  all  theoretical  considerations  aside,  the  facts  of 
the  case,  as  far  as  Europe  is  concerned,  are  as  follows : — A 
cyclone  nearly  always  has  the  highest  temperature  on  the 
right-hand  side  of  the  path  ;  and  for  the  same  distribution 
of  pressure,  there  is  a  considerable  difference  in  the  path 
of  depressions  at  different  seasons  of  the  year,  when  the 
general  slope  of  heat  from  the  equator  to  the  pole  is  not 
the  same. 

Dr.  J.  Y.  Bebber  has  discovered  the  following  relations 
special  for  Germany  and  Central  Europe: — "If  the  dis- 
tribution of  air-pressure  and  temperature  in  the  neighbour- 
hood of  a  depression  are  directed  to  the  same  sense,  then 
the  propagation  of  the  depression  is  nearly  perpendicular 


428  WEATHER. 

to  the  pressure  and  temperature-gradient.  If  the  air- 
pressure  and  temperature  in  the  neighbourhood  of  a 
depression  are  distributed  in  an  opposite  sense,  and  if  the 
differences  are  nearly  equal,  so  is  the  motion  of  the 
depression  checked,  or  even  arrested  (stationary  depres- 
sion), whereby  the  depression  takes  a  long,  more  or  less 
distorted  form,  of  which  the  longer  axis  lies  perpendicular 
to  both  the  air-pressure  and  temperature-gradient.  If, 
with  the  same  distribution  as  before,  either  the  air- 
pressure  or  temperature-gradient  overweighs  on  one  side 
of  the  depression,  so  will  the  direction  of  the  path  be 
determined  by  the  predominating  element.  If  air-pressure 
and  temperature  are  not,  indeed,  opposite,  but  also  not 
distributed  in  the  same  sense  round  the  depression,  so 
will  the  depression  strike  out  a  resultant  direction."  He 
also  thinks  that  pressure  is  the  more  important  deter- 
mination of  cyclone-motion  in  winter,  and  temperature 
the  predominant  influence  in  summer. 

The  conception  of  temperature  and  pressure  gradients 
being  distributed  in  the  same  or  opposite  senses,  appears 
to  be  as  follows : — If  the  highest  pressure  and  highest 
temperature  are  either  both  to  the  north,,  or  both  to  the 
south  of  a  cyclone,  they  are  said  to  be  in  the  same  sense, 
and  the  depression  will  move  at  right  angles  to  both. 
But  suppose  pressure  was  highest  to  north,  and  tempera- 
ture to  south;  then  these  two  elements  would  be  dis- 
tributed in  the  opposite  sense,  and  the  cyclone  would 
probably  be  arrested  in  its  usual  eastward  course. 

These  observations  are  more  suitable  to  Germany 
than  to  Great  Britain,  as  some  of  the  expressions  are 
hardly  applicable  in  the  latter  country,  and  in  England 


FORECASTING  BY  SYNOPTIC  CHARTS.  429 

local  variation  is  so  great,  and  the  area  of  observation  so 
small,  that  the  distribution  of  surrounding  temperature 
can  scarcely  be  used  in  practical  forecasting.  But  in  all 
continental  Europe  we  have  one  practical  rule — that  if 
pressure  is  high  to  the  north  or  north-north-east  of  a 
cyclone,  and  temperature  also  higher  on  that  side  than  to 
the  south,  then  the  propagation  of  the  depression  will 
probably  be  towards  some  point  of  west,  instead  of 
towards  the  east  as  usual.  For  instance,  suppose  we 
found  some  morning  a  cyclone  over  Central  Europe,  with 
an  anticyclone  over  the  North  Sea,  the  natural  presump- 
tion would  be  that  the  depression  would  move — always 
very  slowly  in  this  type — towards  Russia ;  but  if,  as  in 
Figs.  95  and  96,  we  found  the  highest  temperatures  in 
the  Baltic,  and  not  in  Austria,  and  especially  if  the 
temperature  seems  to  rise  to  the  north  or  north-west 
of  the  centre,  then  we  might  forecast  that  the  depres- 
sion would  move,  as  in  this  instance,  westwards  towards 
Great  Britain. 

The  question  how  far  the  cyclone  affects  temperature, 
and  how  far  the  latter  directs  the  former,  will  be  best 
explained  as  follows: — Let  us  call  the  general  slope  of 
temperature  from  land  to  sea,  which  varies  according  to 
the  time  of  year,  the  "seasonal  gradient  of  heat,"  and 
the  patch  of  heat  on  the  right  of  a  cyclone  "  cyclone 
heat ;  "  then  we  may  say  that,  while  the  seasonal  gradient 
has  a  directive  influence  on  the  path  of  the  depression, 
the  cyclone  heat  is  the  product  of  the  moving  whirl 
itself.  The  conclusive  proof  that  the  heat-patch  on  the 
right  front  of  a  depression  belongs  to  the  cyclone  directly, 
and  not  indirectly,  through  the  disturbance  of  radiation, 


430  WEATHER. 

is  found  in  that  peculiar  quality  that  no  thermometer 
can  appreciate,  but  which  is  readily  recognized  by  our 
more  delicate  sensations.  In  a  typical  east-going  cyclone 
the  neuralgic,  pain-producing  heat  comes  with  the  south- 
east wind  on  the  right  front  of  the  depression ;  but  when 
a  cyclone  goes  west,  the  then  right  front  has  a  north-west 
wind  and  the  same  distressing  quality  of  heat. 

FORECASTING  DEPENDS  ON  NO  THEORY. 

We  can  now  readily  understand  from  all  the  fore- 
going remarks  that  forecasting  depends  neither  on  any 
theory  nor  on  any  calculation.  The  whole  science,  from 
beginning  to  end,  rests  solely  on  observation. 

The  shapes  of  isobars,  and  the  relation  of  wind  and 
weather  to  them,  are  matters  of  experience  only.  We 
find  that  certain  kinds  of  weather  are  associated  with 
different  portions  of  each  fundamental  form  of  isobars 
and  we  classify  accordingly.  We  give  each  shape  of 
isobars  a  conventional  name,  but  that  does  not  bind  us 
to  any  theory  of  atmospheric  circulation.  In  like  manner, 
we  see  that  no  averages  or  mean  values  are  of  any  avail 
in  forecasting  weather.  Cyclones  may  usually  take  a 
certain  path,  but  they  need  not  do  so ;  the  greater 
portion  of  the  rainfall  of  any  country  may  come  with  a 
south-west  wind,  but  that  does  not  prevent  many  fine 
days  with  the  wind  from  that  quarter.  On  an  average,  in 
England,  three  days  out  of  four  may  be  cloudy,  and  the 
forecaster  who  always  announced  a  cloudy  day  would  have 
seventy-five  per  cent,  of  success.  Still,  in  an  anticyclonic 
period  his  calculations  would  totally  fail ;  he  could  never 


FORECASTING   BY  SYNOPTIC   CHARTS.  431 

say  what  kind  of  cloud  would  appear,  and  such  a  system 
would  have  no  claim  to  be  called  forecasting  in  the 
modern  sense  of  the  word.  It  is  impossible  to  suppose 
that  we  have  yet  nearly  reached  the  highest  perfection 
of  which  forecasting  is  capable,  but  still  we  know  enough 
of  the  nature  of  the  subject  to  say  with  certainty  that 
calculation  will  never  enter  much  into  the  science  of 
weather-prevision.  Natural  aptitude  and  the  experience 
of  many  years'  study  are  the  qualifications  of  a  successful 
forecaster.  "In  fact,  meteorology  is  not  an  exact,  but 
an  observational  science,  like  geology  or  medicine ;  and 
just  as,  however  accurately  the  symptoms  or  treatment  of 
any  malady  may  be  described,  the  skill  to  recognize  and 
the  judgment  to  treat  must  rest  on  the  ability  of  the 
physician,  so  in  meteorology,  however  carefully  the 
relation  of  weather  to  isobars  may  be  defined  and 
the  nature  of  their  changes  described,  the  judgment 
which  experience  alone  can  give,  to  enable  a  warning  to 
be  issued,  must  ever  depend  on  the  professional  skill  of 
the  forecaster."  » 


DETAIL  POSSIBLE. 

It  may  not  be  out  of  place  to  introduce  here  a  few 
remarks  as  to  the  amount  of  detail  which  it  appears 
possible  to  give  to  daily  forecasts.  Under  various  head- 
ings, we  have  already  discussed  the  influence  of  local 
obstacles  in  modifying  the  appearance  or  intensity  of 
any  kind  of  weather,  and  also  the  powerful  diurnal 
variations  of  every  element  in  all  parts  of  the  world.  When 
to  these  we  add  the  tendency  of  cyclones  to  form  secon- 


432  WEATHER. 

daries,  so  small  as  not  to  show  in  an  ordinary  synoptic 
chart,  then  we  may  easily  understand  that  it  is  the  general 
character  only  of  weather  which  a  forecaster  can  ever 
safely  predict.  The  general  character  is  the  quality  of 
weather  which  we  have  taken  such  pains  to  show  is 
constant  in  each  portion  of  every  shape  of  isobars,  and 
that  never  changes  under  any  local  or  diurnal  variation. 

If  we  live  in  any  place  which  commands  a  view  over 
any  large  tract  of  country,  and  we  think  how  often  we 
see  both  cloud  and  rain  which  only  affect  &  very  small 
portion  of  our  horizon,  we  can  readily  understand  that, 
even  if  it  were  possible  to  issue  minute  forecasts,  every 
few  square  miles  of  country  would  require  a  separate 
warning. 

How  FAR  IN  ADVANCE  CAN  FORECASTS  BE  ISSUED? 

We  may  also  consider  how  far  in  advance  forecasts 
can  safely  be  issued.  The  numerous  charts  which  we 
have  already  given  will  show  the  reader  the  amount  of 
change  which  twelve  or  twenty-four  hours  may  develop 
in  the  distribution  of  pressure.  Sometimes  we  have  been 
able  to  trace  the  changes  in  either  of  these  intervals 
quite  easily;  at  other  times  it  has  been  difficult  to  say 
how  the  first  set  of  isobars  has  grown  into  the  second. 
In  the  United  States  the  observations  are  taken  three 
times  a  day,  and  this  appears  to  be  sufficiently  frequent 
for  all  practical  purposes.  In  most  European  countries, 
reports  are  not  sent  up  more  than  twice  a  day ;  but  with 
this  interval,  cyclones  sometimes  form  so  suddenly  that 
they  are  not  forecast  in  time  to  give  any  warning.  We 


FOKECASTING  BY  SYNOPTIC   CHARTS.  433 

shall  give  an  example  of  such  a  case  further  on  in  this 
chapter. 

Thus,  from  eight  to  twelve  hours  seems  to  be  the 
furthest  time  for  which  forecasts  can  be  issued  in  ad- 
vance, and  even  then  many  local  details  cannot  be  given. 
Some  meteorologists  are  of  opinion  that  a  good  deal  of 
forecasting  will  be  done  in  the  future,  with  the  assistance 
of  a  complete  knowledge  of  recurrent  periods  of  heat, 
cold,  rain,  or  storm ;  and  we  lean  strongly  to  that  view, 
if  these  periods  are  used  in  the  manner  so  fully 
explained  in  our  chapter  on  Seasonal  and  Cyclical 
Periodicities. 

TIME  OF  PREPARATION. 

A  few  particulars  of  the  time  necessary  for  collecting 
and  examining  the  materials  for  synoptic  charts  will 
perhaps  enable  the  public  better  to  understand  the  prac- 
tical conditions  of  the  problem  of  weather-forecasting 
and  storm-warnings. 

In  Great  Britain,  the  morning  observations  are  taken 
at  8  a.m.  Even  with  all  the  rapid  organization  of  the 
British  Post-office,  the  majority  of  the  reports  do  not 
arrive  till  between  9  a.m.  and  10  a.m.  As  fast  as  they 
arrive,  the  information  is  entered  on  a  chart,  and  a 
synoptic  chart  is  constructed.  If  necessary,  telegraphic 
intelligence  of  storms  is  immediately  sent  to  the  coasts, 
and  in  every  case  information  as  to  the  state  of  the 
weather,  and  a  forecast  for  twenty-four  hours  ahead,  is 
sent  to  the  press. 

In  practice,  storm- warnings  can  rarely  be  despatched 

2  F 


434-          .  WEATHER. 

before  11  a.m. ;  that  is  to  say,  three  hours  after  the 
observations  have  been  taken.  If  we  allow  at  least 
another  hour  before  the  public  can  have  access  to  the 
information,  we  see  at  once  that  the  day  is  so  far  gone 
that  the  forecast  can  have  little  practical  importance  for 
the  majority. 

The  greatest  value  is  when  a  storm  has  just  begun 
to  show  over  Valentia  at  8  a.m. ;  then  the  English  coasts 
can  be  warned  in  time.  Still,  in  the  three  or  four  hours 
which  must  elapse  before  the  storm  can  be  warned,  the 
cyclone  will  have  advanced,  perhaps,  as  much  as  a  hundred 
and  twenty  miles,  so  that,  before  a  telegram  can  reach 
the  western  shores  of  England,  the  gale  will  either  have 
commenced,  or  the  appearance  of  the  sky  will  have  given 
unmistakable  warning. 

The  whole  theory  of  storm-warnings  by  means  of 
the  electric  telegraph  is  based  on  the  supposition  that 
the  message  travels  faster  along  the  wire  than  the  storm 
along  the  earth's  surface.  But,  as  the  practical  organi- 
zation of  collection  and  distribution  of  intelligence  takes 
at  least  three  hours,  the  storm  must  either  move  slowly 
or  over  a  considerable  intervening  district  before  any 
set  of  stations  can  be  successfully  warned.  The  forecasts 
which  are  issued  from  reports  taken  at  6  p.m.  are  of 
more  use. 

The  organization  of  the  press  enables  the  public  to 
obtain  the  office  forecast  much  more  quickly  than  by  any 
other  means.  The  British  reports  are  taken  at  6  p.m., 
while  the  United  States  Signal  Office  obtain  their  latest 
about  11  p.m.  These  five  hours  are  an  unquestionable 
gain.  In  Great  Britain  there  are,  however,  difficulties 


FORECASTING  BY  SYNOPTIC  CHARTS.  435 

in  the  way  of  transmission  of  intelligence   during   the 
night. 

Except  in  the  large  towns,  the  majority  of  telegraphic 
stations  are  closed  till  8  a.m.,  so  that  while  the  evening 
forecasts  do  not  reach  them  till  past  eight  o'clock  in  the 
morning,  the  information  for  that  morning  arrives  about 
three  hours  later.  Thus  we  see  the  practical  difficulties 
in  the  way  of  forecasting.  There  is  no  doubt  that  in 
time  some  of  them  will  be  successfully  overcome. 

WHEN  MOST  SUCCESSFUL. 

A  few  remarks  on  the  circumstances  under  which  the 
most  successful  forecasts  can  be  issued  will  also  much 
help  a  general  apprehension  of  the  subject.  We  will 
confine  our  observations  to  Great  Britain  only.  It  is 
very  obvious  that  the  more  striking  the  weather-changes, 
the  more  have  we  something  definite  to  forecast.  When 
we  have  a  well-formed  cyclone,  which  traverses  a  well- 
defined  path,  we  have  strongly  marked  sequences  of  wind 
and  weather,  and  any  error  in  the  forecast  will  only  arise 
from  some  slight  difference  between  the  expected  and 
the  actual  track.  But  when  we  have  what  we  have  seen 
is  the  more  usual  state  of  things  in  Great  Britain— ill- 
defined  depressions  which  move  irregularly,  and  one  or 
more  of  which  fuse  into  a  fresh  cyclone  with  a  new  centre 
— then  we  have  no  definite  sequence  of  weather  to  deal 
with,  but  a  change  which  is  produced  by  the  weather 
at  each  station  gradually  conforming  to  the  varying 
shapes  of  isobars.  The  best  that  can  be  done  then  is  to 
forecast  generally  broken  weather,  and  more  or  less  rain 


436  WEATHER. 

generally;  but  no  attempt  can  be  made  to  foretell  any 
definite  series  of  wind-shifts,  as  in  a  true  cyclone. 

Experience  has  shown  that  in  Great  Britain  no  serious 
gale  has  ever  been  experienced,  unless  there  is  more 
than  half  an  inch  of  difference  of  barometric  pressure 
between  some  two  stations.  Synoptic  charts  will  always 
detect  even  much  smaller  differences;  so  that,  though 
some  uncertainty  will  always  remain  as  to  the  direction 
of  the  wind,  the  force  will  generally  be  at  least  approxi- 
mately forecast  correctly,  except  in  the  case  of  a  very 
sudden  and  unexpected  fall  of  the  barometer. 

Very  different,  however,  is  the  case  of  rain.  Secon- 
daries and  non-isobaric  rains  are  the  forecaster's  bugbear ; 
they  form  so  quickly,  show  so  little  on  a  synoptic  chart, 
and  move  so  irregularly,  that  rain  in  general  terms  is  all 
that  the  forecaster  can  usually  say.  In  summer,  when 
he  sees  the  characteristic  loops  in  the  isobars  which  con- 
stitute secondaries,  he  can  safely  predict  thunder  and 
rain ;  but  he  cannot  attempt  to  localize  either  of  these 
phenomena. 

Sometimes,  too,  secondaries  are  so  small  that  they  do 
not  show  at  all  on  a  synoptic  chart,  which  is  constructed 
on  reports  received  from  stationsoften  a  hundred  and  fifty 
or  two  hundred  miles  apart.  The  whole  loop  of  a 
secondary  need  not  be  nearly  so  large ;  and  then  a  depres- 
sion of  that  class  might  lie  between  two  stations,  and  yet 
be  indicated  at  neither.  The  weather,  however,  would  be 
profoundly  modified,  and  the  forecasts  would  probably  be 
erroneous. 

There  is  also  always  the  important  difference  between 
wind  and  rain,  that  the  former  is  always  in  the  main 


FORECASTING  BY  SYNOPTIC   CHARTS.  437 

•determined  by  the  steepness  of  the  gradients,  while  the 
amount  of  precipitation  bears  no  relation  to  any  known 
meteorological  element. 

In  many  shapes  of  isobars  we  know  that  there  will  be 
rainfall,  but  whether  much  or  little,  we  cannot  tell  at 
present. 

From  these  considerations  we  need  not  be  surprised  to 
find  that  in  all  offices,  except  in  Japan,  wind  is  better 
forecast  than  rain. 

SOURCES  OF  FAILURE. 

From  the  conditions  of  successful  forecasts,  we  can 
readily  turn  to  those  of  unsuccessful  predictions.  Besides 
the  uncertainty  of  rainfall  due  to  the  action  of  secondaries, 
there  are  four  principal  sources  of  failure :  the  sudden 
formation  of  an  intense  cyclone  ;  the  sudden  dying  out  of 
an  existing  cyclone;  the  motion  of  a  cyclone  in  an  un- 
expected path ;  and,  lastly,  an  error  in  the  judgment  of 
the  forecaster. 

In  the  first  case,  of  the  sudden  formation  of  a  new 
cyclone,  the  whole  forecast  is  necessarily  totally  upset, 
and  the  weather  which  is  experienced  is  worse  than  had 
been  anticipated. 

The  converse  occurs  when  an  intense  cyclone  suddenly 
dies  out.  Then  the  weather  is  much  better  than  was 
expected ;  but  neither  in  this  case  nor  in  the  preceding 
one  can  settled  weather  be  expected. 

When  a  cyclone  takes  an  unusual  path,  the  general 
character  of  the  weather  will  remain  bad,  but  the  direc- 
tion of  the  wind  and  the  details  in  different  districts  will 


438  WEATHER. 

be  wrongly  forecast.  We  have  already  given  instances 
of  cyclones  which  move  in  no  well-defined  path,  and  more 
complicated  cases  often  occur.  Sometimes  the  path  will 
describe  a  complete  circle  of  no  very  great  diameter ;  but 
the  commonest  case  in  Western  Europe  is  when  the  path 
of  a  cyclone  takes  the  form  of  the  letter  V.  For  instance, 
a  cyclone  comes  in  from  the  Atlantic  from  about  due 
west,  and  after  it  has  gone  as  far  as  England,  it  moves 
back  again  in  a  north- westerly  direction,  as  it  has  not 
been  able  to  pass  the  area  of  high  pressure  which  would 
then  be  lying  over  Northern  and  Central  Europe.  In 
another  common  case,  the  cyclone  comes  down  from  the 
north-west  on  to  England,  and  then  passes  off  in  a  north- 
easterly direction  towards  Norway.  In  all  such  cases  the 
forecaster  is  at  a  great  disadvantage. 

Lastly,  the  judgment  of  the  forecaster  will  sometimes 
err.  We  have  shown  that  no  absolute  law  of  cyclone- 
motion  can  be  laid  down,  and  that,  in  fact,  the  tracking 
of  well-defined  depressions  forms  but  a  small  portion  of 
the  forecaster's  business.  On  the  larger  number  of  days 
he  has  to  estimate  how,  or  where,  cyclones  will  form  in 
an  ill-defined  area  of  low  pressure,  or  how  far  an  area  of 
low  pressure  will  encroach  on  another  region  of  high 
barometer.  In  this,  he  must  rely  on  his  own  opinion  and 
experience  alone;  that  must  be  fallible  sometimes,  but 
better  results  are  obtained  by  trusting  to  personal  skill 
than  by  attempting  to  use  any  mechanical  rules  or 
maxims. 

Men  differ  in  their  aptitude  to  forecast  weather  in  the 
same  way  as  physicians  differ  as  to  the  accuracy  of  their 
diagnosis;  but  just  as  the  best  results  are  obtained  by 


FORECASTING,  BY   SYNOPTIC   CHARTS.  439 

selecting  the  doctor  whom  experience  has  shown  to  be  the 
most  successful  practitioner,  so  the  best  forecasts  are  got 
by  selecting  the  meteorologist  who  has  been  the  most 
successful  in  that  branch  of  the  subject.  In  the  United 
States  Signal  Office  at  the  present  time,  four  men  take  the 
duty  of  forecasting  in  rotation.  They  have  so  far  all  been 
ground  in  the  same  mill,  by  passing  through  a  two- 
years'  course  of  the  same  hard  training ;  and  it  is  found  in 
practice  that  the  difference  between  the  best  and  worst  is 
two  per  cent,  in  the  number  of  successful  forecasts.  For 
instance,  if  the  best  man  gets  ninety  per  cent.,  the  worst 
will  attain  to  eighty-eight  per  cent,  of  success. 

SOME  COUNTRIES  EASIER  THAN  OTHERS. 

From  all  that  we  have  now  explained,  it  will  be  very 
evident  that  forecasting  is  much  easier  in  some  countries 
than  others.  In  the  tropics,  the  great  seasonal  changes 
come  on  regularly,  and  the  smaller  changes  from  day  to 
day  are  insignificant.  In  the  two  or  three  days  of  any 
year  on  which  a  regular  cyclone  may  form,  the  pre- 
monitory symptoms  are  so  obvious  that  there  is  no  diffi- 
culty in  framing  a  forecast. 

In  temperate  regions,  those  countries  will  be  the  best 
situated  which  lie  to  the  east  of  a  well-observed  land  area, 
because  most  disturbances  in  the  temperate  zone  move 
from  the  west. 

Thus  Germany  and  Norway  are  much  more  favourably 
located  for  weather-prevision  than  either  England  or 
France. 

In  the  vear  1869  twentv-three   storms  were  felt  in 


440  WEATHER. 

Hamburg,  and  of  these  twenty-two  had  previously  passed 
over  some  part  of  Great  Britain.  In  the  seven  years, 
1867-1874,  301  warning  messages  were  issued  from 
London  to  Hamburg;  seventy-two  per  cent,  of  these 
warnings  were  followed  by  gales,  while  in  only  three 
cases  did  the  storm  outrun  the  message.  Then  in  the 
United  States,  the  majority  of  cyclones  commence  in  the 
Kocky  Mountains ;  so  that  with  the  admirable  organization 
of  the  Signal  Office,  timely  warning  of  serious  gales  can 
usually  be  sent  to  the  Eastern  States  of  the  Union. 

Great  Britain  is  situated  in  a  region  of  peculiar  diffi- 
culty. Not  only  does  her  insular  position  preclude  any 
early  knowledge  of  the  advent  of  cyclones,  but,  from  the 
nature  of  weather-types,  she  is  more  exposed  to  unsettled 
weather  than  any  other  part  of  Europe. 

We  have  seen  in  our  chapter  on  Weather-Types,  that 
the  positions  of  the  great  areas  of  high  and  low  pressure 
are  to  a  certain  extent  determined  by  the  areas  of  land 
and  water. 

When  the  persistent  anticyclone  of  the  southerly  type 
lies  over  Scandinavia,  the  Atlantic  is  covered  by  low 
pressure  and  bad  weather;  when  the  great  anticyclone 
covers  the  Atlantic  in  the  northerly  type,  then  pressure 
is  lowest  and  weather  worst  in  Scandinavia ;  so  that,  in 
almost  every  case,  Great  Britain  is  on  the  boundary 
between  a  cyclonic  or  anticyclonic  system,  and  is  there- 
fore exposed  to  changeable  weather.  Just  as  an  outlying 
rock  is  exposed  to  the  wash  of  every  sea,  so  England  is 
exposed  to  the  disturbing  influences  of  every  type  of 
European  or  Atlantic  bad  weather. 


FORECASTING  BY  SYNOPTIC  CHARTS.  441 

EXAMPLES  OF  ACTUAL  FORECASTS. 
BRITISH. 

After  these  explanations  we  will  now  give  some 
examples  of  actual  forecasts  in  different  countries,  com- 
mencing with  Great  Britain.  The  latter  are  taken,  with 
some  important  additions,  from  the  author's  work  on  the 
principles  of  forecasting  before  mentioned.  We  have 
selected  our  first  example  to  illustrate  a  completely  suc- 
cessful forecast  which  depended  on  the  estimate  of  the 
forecaster  as  to  the  progress  of  an  ill-defined  area  of  low 
pressure  towards  the  east.  This  is  one  of  the  commonest 
c*ases  which  occur  in  Great  Britain.  The  chief  points 
which  the  forecaster  had  to  consider  were  the  direction  in 
which  the  depression  would  move,  and  especially  how  far 
east  it  would  pass  without  being  arrested  in  its  progress. 
Also,  whether  the  gradients  would  become  sufficiently 
steep  to  give  rise  to  serious  gales. 

But  to  understand  properly  the  details  of  the  warn- 
ings, we  must  first  explain  the  districts  into  which  the 
United  Kingdom  is  divided  for  the  localization  of  weather- 
forecasts. 

In  Fig.  92  we  give  a  map  of  the  eleven  districts  in  the 
British  Islands  which  are  separately  warned ;  and  by 
ineans  of  this  map  the  subsequent  details  will  be  easily 
followed. 

A  glance  at  the  relative  size  of  any  one  of  these 
districts  and  the  area  covered  by  even  a  small  cyclone, 
will  show  at  once  how  much  a  small  change  in  the  cyclone 
may  mar  the  most  carefully  drawn  deductions  of  the  fore- 


442 


WEATHER. 


caster ;  the  smallest  loop  in  the  isobars  which  we  saw  so 
often  in  our  large  charts  of  weather-types  would  entirely 
alter  the  details,  though  not  the  general  character  of  the 
weather  which  would  be  experienced.  The  action  of  such 
a  secondary  might  reduce  the  force  of  the  wind  so  much 
that  some  district  would  receive  a  warning  which  was  not 


FIG.  92. — British  forecasting  districts. 

justified  by  the  event,  or  develop  rain  where  fine  weather 
had  been  anticipated  and  forecast. 

In  the  left-hand  portion  of  Fig.  93  we  give  the  chart 
from  which  forecasts  had  to  be  issued  at  8  a.m.,  February  6, 
1883.  We  see  in  it  at  once  the  commonest  features  of 
the  southerly  type  of  weather  with  the  pressure  high  over 
Scandinavia  and  low  over  the  west  of  Ireland,  while  the 
isobars  run  nearly  due  north  and  south.  Southerly  gales 
have  already  commenced  in  the  west  and  north,  while  fine 


FORECASTING  BY  SYNOPTIC   CHARTS. 


weather  prevails  over  the  south  and  east  coasts  of  Great 
Britain. 

It  was  also  known,  by  comparison  with  the  previous 
charts,  that  while  the  barometer  was  rising  over  Norway, 
it  was  falling,  but  only  slowly,  over  the  western  coasts  of 
Ireland.  Now,  from  all  that  we  have  already  explained 


6.2.83 

8.  a.m. 


FIG.  93.— Successful  forecast  (British). 

as  to  the  nature  of  this  type,  it  is  evident  that  there  is 
no  fear  of  the  depression  crossing  England  so  as  to  bring 
tiny  great  change  of  wind,  but  that  the  gradients  will  get 
steeper  for  southerly  winds  with  bad  weather,  and  that 
probably  the  south  and  east  coasts  will  not  be  affected. 
Then,  as  to  storm-warnings,  all  the  north  and  west  (Dis- 
tricts 0,  1,  6,  and  9)  were  already  warned,  but  as  the  south 
of  Ireland  (District  10)  will  be  affected  by  the  increasing 


444 


WEATHER. 


gradients,  warnings  are  now  necessary  for  it  also.  Hence 
the  following  forecasts  were  issued  to  the  different  dis- 
tricts : — 

FORECASTS  FOR  THE  TWENTY-FOUR  HOURS  ENDING  AT  NOON  ON 
FEBRUARY  7,  1883. 


Districts. 


Forecasts. 


0.  Scotland,  N. 

1.  Scotland,  E. 

2.  England,  N.E. 

3.  England,  E. 

4.  Midland  Counties  ... 

5.  England,  S.,  and  the 

Channel 

6.  Scotland,  W. 


7.  England,  N.W. 

8.  England,  S.W. 

9.  Ireland,  N. 


10.  Ireland,  S.  ... 
Warnings 


Southerly  strong  winds  and  gales;  cloudy 
generally,  with  some  rain. 

Do.  Do. 

South-easterly  winds,  moderate  inland,  strong 
on  coast ;  fair  generally. 

Do.  Do. 

Same  as  No.  5. 

South-easterly  and  southerly  winds,  mode- 
rate or  fresh  ;  fair  generally. 

South-easterly  and  southerly  strong  winds, 
perhaps  a  gale  j  fair  to  cloudy,  and 
unsettled. 

Do.  Do. 

South-easterly  and  southerly  winds,  in. 
creasing ;  cloudy. 

South-easterly  and  southerly  winds,  in- 
creasing to  a  gale ;  cloudy,  unsettled ; 
some  rain. 

Do.  Do. 

The  south  cone  is  still  up  in  Districts  0,  6,  9, 
and  parts  of  1  and  7,  and  has  been  re- 
hoisted  this  morning  in  District  10. 


By  looking  at  the  right-hand  portion  of  the  chart 
{Fig.  93)  for  6  p.m.  on  the  same  day,  we  find  that  the 
above  anticipations  have  been  completely  verified.  Wind 
-and  rain  have  increased  in  the  west  and  north,  but  in 
South-east  England  the  weather  remains  fine.  In  his 
journal  near  Dover,  in  District  5,  on  that  day,  the  author 
finds  the  following  entry :—"  February  6,  1883.— Cold, 


FORECASTING  BY    SYNOPTIC   CHARTS. 


445 


dry,  very  fine  and  bright ;  wind  south-east,  fresh."  Hence 
the  forecasts  were  a  complete  success.  The  weather  was 
cool  near  Dover  because  that  town  was  under  the  influence 
of  the  European  anticyclone  ;  but  in  all  the  western  dis- 
tricts temperature  was  very  high  for  the  season.  In  the 


FIG.  94. — Failure  of  forecasts. 

selection  of  this  example  we  had,  however,  an  additional 
object,  viz.  to  illustrate  what  we  have  laid  down  relative 
to  the  use  of  periodicities  in  forecasting. 

We  have  already  mentioned  that  the  period  February 
7-10  is  one  of  recurrent  cold  weather,  whence,  if  the  fore- 
caster had  trusted  blindly  to  periodicities,  he  would  have 
made  a  complete  failure.  On  the  other  hand,  had  he 
discovered  on  this  day  the  commencement  of  either  the 
northerly  or  easterly  types,  the  knowledge  of  the 
periodicity  would  have  been  of  great  use  to  him. 


446 


WEATHER. 


Our  next  illustration  will  be  that  of  a  kind  which, 
fortunately,  rarely  occurs,  viz.  the  sudden  formation  of 
a  cyclone  in  an  unexpected  position,  which  entirely  upsets 
all  forecasting.  In  the  left-hand  portion  of  Fig.  94  we 
give  a  chart  for  6  p.m.,  October  23,  1882.  There  we  see 
the  most  familiar  features  of  the  westerly  type  of  weather, 
and  though  the  barometer  was  falling  over  the  Bay  of 
Biscay,  and  rising  over  Scotland,  there  was  no  reason  to 
•expect  that  the  ordinary  sequence  of  that  kind  of  weather 
would  be  disturbed — that  is  to  say,  that  west  and  south- 
west winds,  with  rather  showery  weather,  would  prevail. 
Accordingly  the  following  forecasts  were  issued : — 


FORECASTS  OF  WEATHER  FOR  OCTOBER  24, 1882,  ISSUED  AT  8.30  P.M. 

THE    PREVIOUS   DAY. 


Districts. 


Forecasts. 


0.  Scotland,  N. 

1.  Scotland,  E.      ... 

2.  England,  N.E. 

3.  England,  E. 

4.  Midland  Counties 
.5.  England,  S 


6.  Scotland,  W. 

7.  England,  N.W.  ... 

8.  England,  S.W. 

9.  Ireland,  N. 

10.  Ireland,  S.  ... 
Warnings 


South-westerly  breezes,  fresh  or  moderate ; 
showery. 

South-westerly  breezes ;    moderate  j    some 
showers,  with  bright  intervals. 
Do.  Do. 

Same  as  No.  5. 

Same  as  No.  1. 

Westerly  and  south-westerly  breezes,  light 
to  fresh ;  fine  and  cold  at  first,  some  local 
showers  later. 

Same  as  No.  0. 

Same  as  No.  0. 

South-westerly  winds,  fresh  to  strong; 
showery. 

Wind  returning  to  south-west,  and  freshen- 
ing ;  weather  showery. 

Do.  Do. 

None  issued. 


FOEECASTING  BY   SYNOPTIC   CHAETS.  447 

When  we  come  to  look,  however,  at  the  right-hand 
chart  in  the  figure  for  8  a.m.  the  following  morning,  we 
find  that  a  small  well-defined  cyclone  had  formed  during 
the  night  over  the  English  Channel,  which  moved  during 
the  day  towards  north-north-east,  and  thereby  produced 
continuous  rain  with  complete  shifts  of  the  wind  through 
180°  in  many  parts  of  the  country,  so  that  the  forecasts 
issued  were  a  complete  failure. 

Present  Results. 

It  will  now  be  interesting  to  give  some  idea  of  the 
amount  of  success  which  at  present  attends  both  every- 
day weather-forecasts  and  also  storm-warnings,  as  issued 
by  the  British  Meteorological  Office  for  every  district ; 
each  forecast  being  considered  under  the  separate  headings 
of  "  Wind  "  and  "  Weather,"  and  the  amount  of  success 
or  failure  is  divided  into  four  degrees — complete  success, 
partial  (more  than  half)  success,  partial  failure,  and  total 
failure.  In  practice  it  is  found  that  the  percentage  of 
any  district  varies  but  little  from  year  to  year,  though, 
on  the  whole,  there  is  a  slow  progressive  improvement. 
The  subjoined  summary  of  weather-forecasts  for  the  year 
ending  March  31,  1882,  may,  therefore,  be  taken  as  a  fair 
sample  of  the  results  usually  attained  by  the  Meteorological 
Office. 


448 


WEATHER. 

SUMMARY  OF  EESULTS. 


Percentages. 

Total 

District. 

pprcentage 

Complete 

Partial 

Partial 

Total 

of  success. 

Success. 

success. 

failure. 

failure. 

Scotland,  1ST  

39 

42 

14 

5 

81 

Scotland,  E  

35 

43 

15 

7 

78 

England,  N.E  

32 

46 

17 

5 

78 

England,  E. 

33 

44 

17 

6 

77 

Midland  Counties     ... 

31 

46 

18 

5 

77 

England,  S  

35 

46 

14 

5 

81 

Scotland,  W  

30 

44 

19 

7 

74 

England,  N.W.      ... 

32 

44 

17 

7 

76 

England,  S.W  

34 

42 

18 

6 

76 

Ireland,  N  

36 

44 

14 

6 

80 

Ireland,  S  

35 

41 

16 

8 

76 

Summary  

34 

44 

16 

6 

78 

By  this  it  will  be  seen  that  the  complete  or  partial 
successes  amount  to  seventy-eight  per  cent.,  varying  from 
seventy-four  per  cent,  in  the  west  of  Scotland  to  eighty- 
one  per  cent,  in  the  north  of  Scotland  and  south  of 
England. 

Checking  Forecasts. 

It  might  appear  at  first  sight  that  when  a  forecast 
had  been  issued,  it  would  be  the  simplest  thing  possible 
to  check  it,  and  to  say  whether  it  had  been  successful 
or  not. 

In  practice,  however,  it  is  very  different,  as  will  be 
seen  from  the  following  remarks.  The  difficulty  arises 
from  two  sources — the  local  variation  of  wind  and  rain  in 
the  same  district,  and  the  difficulty  of  assigning  a 


FORECASTING  BY"  SYNOPTIC   CHARTS.  449 

mechanical  measure  to  such  elements  as  a  gale  of  wind 
or  a  rainy  day. 

For  instance,  some  of  the  British  forecasting  districts 
are  about  two  hundred  miles  by  one  hundred,  and  contain, 
two  or  three  hundred  square  miles.  Even  within  this 
limited  area  considerable  differences  of  weather  may  be 
experienced.  It  may  blow  a  gale  at  Dover,  and  only  a 
fresh  breeze  in  London,  though  those  towns  are  only 
seventy  miles  apart.  The  difficulties  are  even  greater 
when  we  come  to  treat  the  British  Islands  as  a  whole. 

For  instance,  suppose  we  want  to  test  the  truth  of  the 
popular  saying  as  to  the  frequency  of  gales  at  the 
equinox,  hoAV  are  we  to  define  what  is  a  gale?  Is  it 
enough  to  prove  the  saying  if  a  gale  has  been  experienced 
in  only  one  of  the  eleven'  districts,  or  must  we  report  a 
gale  from  three  or  four  districts  at  least,  before  we  can 
say  that  a  storm  swept  over  Great  Britain  about  such  or 
such  a  date  ?  It  follows  from  these  general  considerations 
that  the  total  success  which  is  credited  to  any  district 
will  always  be  much  better  than  if  the  records  at  any 
one  station  had  been  compared  with  forecasts  issued  to 
the  district  in  which  it  lay.  Every  office  checks  its  own 
forecasts  by  its  own  method,  so  that  the  relative  per- 
centages of  success  which  we  shall  give  hereafter  cannot 
be  strictly  compared.  They  are,  however,  very  good 
approximations  to  the  truth. 

GERMAN. 

We  will  pass  from  the  consideration  of  winter  and 
autumn  gales,  which  move  in  an  easterly  direction,  to  the 

2  G 


450 


WEATHER. 


very  different  state  of  things  which  brings  thunder  and 
rain  to  Central  Europe  during  the  summer  months.  We 
have,  therefore,  selected  an  illustration  of  a  partially 
successful  set  of  forecasts  issued  by  the  Deutsche  Seewarte 
at  Hamburg  on  August  13  and  14,  1880.  This  will  be 
a  very  typical  example  of  rain  with  secondaries,  and  of 
the  apparent  independence  of  rain  on  the  barometer.  In 
Fig.  95  we  give  a  synoptic  chart  over  the  greater  portion 


JO.  I 


FIGS.  95  and  96. — Partially  successful  forecast  (Germany). 

of  Central  Europe  at  8  a.m.,  Hamburg  time;  and  in 
Fig.  96  a  similar  chart  for  the  succeeding  morning.  The 
broad  features  of  these  two  days  is  very  simple.  An  anti- 
cyclone rests  over  Great  Britain,  while  a  shallow  cyclone 
is  moving  westwards  up  the  middle  valley  of  the  Danube. 
When  we  come  to  look  at  the  movement  of  the  isobars 
between  the  first  and  second  day,  we  find  that  the  position 
of  the  line  of  29*9  ins.  (750  mm.)  has  scarcely  altered,  but 
that  the  isobar  has  become  looped  up  into  secondaries. 

The  result  of  all  this  on  the  weather  was  to  produce 
rain  and  thunder  with  very  little  wind,  and  insignificant 
changes  in  the  reading  of  the  barometer  at  any  station. 


FORECASTING  BY  SYNOPTIC  CHARTS.  451 

The  following  forecasts  were  issued  from  the  office  of 
the  Deutsche  Seewarte  at  Hamburg  for  these  two  days : — 

"Prospects  for  the  weather  of  August  14,  1880,  in 
Germany. — General.  Continuance  of  the  changeable 
weather,  with  precipitation,  and  light  to  fresh  wind — in 
the  north,  mostly  northerly ;  in  the  south,  mostly  westerly 
•to  northerly,  with  a  temperature  little  changed  or  else 
falling.  Here  and  there  thunderstorms. 

"Prospects  for  the  weather  of  August  15,  1880,  in 
•Germany. — General.  Bather  warm — in  the  west  for  the 
most  part  bright  weather ;  in  the  east  overcast  weather 
prevalent,  with  light  wind.  Inclination  to  thunder." 

The  cyclone  which  we  see  on  our  first  chart  lying 
over  Hungary  had  been  moving  slowly  for  two  or  three 
previous  days  up  the  valley  of  the  Danube  river,  and  the 
above  forecasts  are  evidently  based  on  the  supposition 
that  the  motion  of  the  depression  will  continue  in  the 
same  direction.  The  forecaster  who  is  skilled  in  the 
meteorology  of  Germany  knows  both  the  kind  of  cyclone 
which  moves  westwards  and  also  the  kind  that  will 
develop  rain  and  thunder,  but  he  cannot  tell  exactly 
where  the  rain  will  be  heaviest,  nor  whether  the  intensity 
will  increase. 

For  this  latter  reason  the  above  forecasts  do  not 
sufficiently  indicate  the  very  heavy  rain  and  disastrous 
floods  which  occurred  in  Austria  and  South  Germany 
during  the  period  in  question. 

J.  Hann  (LXXXII.  Bunde  Accad.  d.  Wiss.  II.  Abbth. 
Nov.,  1880)  has  made  the  weather  of  this  period  the 
subject  of  a  special  memoir.  He  finds  the  following  dates 
for  the  heaviest  rainfall: — August  11,  Siebenburg  and 


452  WEATHER. 

South-East  Hungary;  12th,  all  Hungary,  Schlesien, 
Nieder  Ostereich ;  12th  and  15th,  Ober  Ostereich,  east  of 
South  Bavaria ;  13th,  west  of  South  Bavaria,  Bohemia, 
Saxon  Erzgebirge ;  14th,  North  Tyrol  and  Pinzgau ; 
15th,  second  maximum,  Salzkammergut,  West  Schlesien, 
JSTorth  Bohemia.  That  is  to  say,  that  on  the  whole  the 
position  of  greatest  rainfall  travelled  westwards  with  the 
primary  cyclone.  Some,  but  not  all,  of  the  rain  was 
accompanied  by  thunderstorms.  His  investigations  were 
mostly  from  the  point  of  view  which  connects  rain  with 
the  motion  of  the  barometer,  as  observed  at  any  one 
station.  The  results  which  he  obtains  are  most  striking 
illustrations  of  the  principle  we  have  so  often  alluded  to, 
that  the  rain  of  secondaries  is  out  of  all  proportion  to  the 
barometric  changes  as  recroded  by  a  solitary  observer; 
and  that  the  position  of  heaviest  rainfall  cannot  be  given 
from  an  inspection  of  the  isobars,  as  in  the  case  of 
primary  cyclones. 

The  conclusion  which  he  arrives  at  as  regards  these 
two  points  are  as  follows  : — 

"  The  appearance  of  a  barometer  minimum  in  Hungary 
occasioned  enormous  and  extended  precipitation  on  the 
west  and  north-west  sides  of  this  barometer  depression. 
A  reaction  of  this  precipitation  on  the  position  of  the 
centre  of  the  depression  is  scarcely  perceptible. 

"  Also  the  general  distribution  of  pressure  (the  form 
of  the  isobars)  shows  no  relation  to  the  area  of  the 
intense  precipitation. 

"  We  find,  therefore,  through  the  investigation  of  the 
relative  lowest  barometer  reading  in  its  behaviour  t& 
rainfall,  that  our  former  conclusions  are  confirmed. 


FORECASTING  BY   SYNOPTIC   CHARTS.  453 

"  A  relation  between  barometer  change  and  rainfall  is 
scarcely  obvious,  and  the  conclusion  is  justified  that  the 
barometer  fall,  in  the  first  instance,  does  not  depend  upon 
rainfall,  and  especially  is  not  perceptibly  influenced  by 
the  last." 

Seewarte  Success. 

The  following  is  the  result  of  the  tests  of  the  general 
(allgemein)  weather-forecasts  published  by  the  Deutsche 
Seewarte  at  Hamburg  in  1882.  The  total  percentage  of 
success  is  credited  with  half  the  partial  success. 


(Weather          ...  ...     78  per  cent. 

Wind       ...  ...             75         „ 

Temperature    ...  ...     78         „ 

(  Success    ...  ...             69         „ 

fipnpvnl                        Partial  success  ...     15 

leial Failure 15 

I  Total  of  success  '                  77 


UNITED  STATES  FORECASTS. 

We  will  now  give  some  illustration  of  forecasts  and 
results  in  the  United  States  and  Canada.  As  an  example 
of  successful  forecasting,  let  us  look  back  at  Figs.  30-35, 
in  which  we  gave  very  detailed  charts  of  the  wind  and 
weather  in  the  United  States  on  January  20  and  21, 1873. 

Figs.  31  and  34  give  the  isobars,  wind,  and  weather 
on  the  21st,  4.35  p.m.,  Washington  time.  The  cyclone 
which  we  found  there  over  the  Middle  States  had 
travelled  in  an  east-north-east  direction  since  morning,  as 
we  see  by  reference  to  the  preceding  chart  (Fig.  30). 

The  subjoined  forecast  was  evidently  based  on  the 


454  WEATHER. 

idea  that  the  cyclone  would  continue  to  move  in  the 
same  direction,  so  as  to  pass  over  the  New  England  States, 
and  that  the  Eocky  Mountains  cyclone  would  develop 
and  advance  eastwards.  That  is  to  say,  that  in  the  New 
England  States  the  wind  would  shift  to  the  north  and 
west  with  a  rising  barometer,  falling  thermometer,  and 
clear  sky  of  the  rear  of  a  cyclone ;  that  in  the  Middle 
States  somewhat  similar  weather  would  be  experienced ; 
but  that  from  Tennessee,  northwards  over  Ohio,  the  wind 
would  shift  to  the  south  and  east,  with  a  rising  temperature 
and  cloudy  sky,  from  the  action  of  the  front  of  the  new 
cyclone. 

In  the  result,  by  11  p.m.  the  same  day  (see  Figs.  32 
and  35),  the  first  cyclone  moved  as  expected,  and  the 
forecasts  were  a  complete  success  in  the  New  England 
and  Middle  States. 

The  new  cyclone  did  not,  however,  advance  as 
anticipated,  and  the  southern  anticyclone  increased  in 
size.  Hence,  from  Tennessee,  northward  over  Ohio,  though 
some  south  and  east  wind  was  experienced,  the  weather 
remained  fine,  and  the  temperature  fell  from  the  radiation 
of  the  anticyclone,  instead  of  rising  for  the  cyclone. 
Hence  the  forecast  was  only  partly  verified.  Nothing 
could  show  more  clearly  the  difficulties  of  a  forecaster 
than  this  example.  He  was  unquestionably  justified  in 
expecting  the  advance  of  the  new  cyclone,  but  he  was 
baffled  by  one  of  the  endless  shifts  which  accompany  the 
growth  of  cyclones. 

Turn  now  to  the  charts  for  the  next  day,  which  we 
gave  in  Figs.  42-44,  to  illustrate  the  nature  of  diurnal 
temperature- variations.  There  we  find  by  11  p.m.  that 


FORECASTING  BY  SYNOPTIC  CHARTS.  455 

day  (see  Fig.  42)  the  second  cyclone  had  advanced  very 
much  as  had  been  expected,  only  more  slowly.  The 
increase  of  the  anticyclone  over  the  Southern  States  on 
the  first  day  was  due  to  that  gathering-up  pressure  which 
we  have  seen  so  often  precedes  the  full  development  of 
an  incipient  cyclone,  but  could  not  have  been  forecast  in 
our  present  state  of  knowledge. 

The  most  unsuccessful  portion  of  the  forecast  related 
to  the  weather  in  the  north-west.  The  probabilities  were 
given  for  "  winds  shifting  to  northerly  and  westerly,  with 
rising  barometer,  falling  temperature,  and  clearing  but 
partly  cloudy  weather."  This  was  based  on  the  sup- 
position that  the  new  cyclone  would  be  small  and  follow 
nearly  the  track  of  the  preceding  depression.  The  pre- 
diction was  not  justified  by  the  result,  but  shows  very 
clearly  the  scope  of  individual  judgment.  The  follow- 
ing is  an  exact  copy  of  the  published  synopsis  and 
probabilities : — 

"  Washington  D.C. 
"  Tuesday,  January  21,  1873,  4-35  p.m. 

"  Synopsis. 

"The  barometer  has  continued  falling,  with  rising 
temperature  from  Florida  to  the  Middle  and  New  Eng- 
land States,  the  lowest  being  central  over  the  Lower 
Lake  region,  where  fresh  and  brisk  variable  winds  and 
rain  and  snow  are  now  prevailing.  Cloudy  weather,  rain, 
and  fresh  and  brisk  southerly  to  easterly  winds  are  now 
prevailing  from  North  Carolina  to  New  York  and  New 
England,  excepting  light  snow  over  northern  part  of 
latter.  Generally  clear  weather  from  South  Carolina  to 


456  WEATHER. 

Tennessee,  and  southward  to  the  Gulf.  Westerly  to 
northerly  winds,  cloudy  weather,  light  snow,  and  falling 
temperature  from  Kentucky  to  the  Upper  Lakes  and 
Lake  Erie.  The  rivers  have  fallen  at  Pittsburgh  and 
Cairo,  but  reported  to  have  risen  over  five  feet  at 
Cincinnati. 


"  Probabilities. 

"For  New  England,  winds  shifting  to  northerly  and 
westerly  on  Wednesday,  with  falling  temperature,  rising 
barometer,  and  clearing  weather,  accompanied  by  occa- 
sionally light  snow.  For  South  Atlantic  and  Middle 
States,  rising  barometer,  fresh  to  brisk  westerly  to 
northerly  winds,  and  clear  and  clearing  weather,  with 
falling  temperature  over  latter,  and  possibly  areas  of 
light  snow  over  northern  portion.  For  Gulf  States, 
falling  barometer,  somewhat  higher  temperature,  south- 
easterly and  southerly  winds,  and  increasing  cloudiness. 
with  possibly  threatening  weather.  From  Tennessee, 
northward  over  Ohio  and  southern  portions  of  Michigan 
and  Wisconsin,  winds  shifting  to  southerly  and  easterly, 
rising  temperature,  cloudy  weather,  and  possibly  light 
rain.  For  Northern  portions  of  Michigan  and  Wisconsin, 
easterly  to  northerly  winds,  cloudy  weather,  and  snow. 
For  the  North-west,  winds  shifting  to  northerly  and 
westerly,  with  rising  barometer,  falling  temperature,  and 
clearing  but  partly  cloudy  weather.  A  portion  of  the 
afternoon  telegraphic  reports  from  Minnesota  and  Dakota 
are  missing. 

"  Facts — 11  p.m.  (following  the  above  '  Probabilities '). 


FORECASTING  BY  SYNOPTIC   CHAKTS.  457 

"  Wind  and  Weather. 

"1.  Clear. — At  Augusta,  Mobile,  and  Montgomery, 
cairn ;  San  Diego,  wind  north-east,  light ;  Memphis, 
Nashville,  Baltimore,  Virginia  City,  and  Washington, 
wind  west,  light ;  Norfolk,  Wilmington,  Charleston,  and 
Savannah,  wind  south-west,  fresh;  New  Orleans,  wind 
south-east,  fresh  ;  Denver,  wind  north-west,  fresh ;  Corinne, 
wind  north,  fresh. 

"  2.  Fair. — At  Keokut  and  Saint  Louis,  calm ;  Shreve- 
port,  wind  south-east,  light ;  New  York,  wind  south-west, 
gentle;  Philadelphia,  wind  west,  gentle;  Milwaukee, 
wind  north-west,  gentle ;  Lynchburg,  wind  south-west, 
fresh ;  Cairo  and  G-alveston,  wind  south-east,  fresh. 

"3.  Cloudy. — At  Burlington,  Chicago,  and  Oswego, 
calm;  Portland,  Oreg.,  wind  north-west,  light;  Daven- 
port, wind  north-east,  light ;  St.  Paul  and  Sangeen,  wind 
north-east,  gentle ;  Louisville,  wind  south-west,  gentle ; 
Leaven  worth,  wind  south-east,  gentle ;  Kochester,  Toledo, 
and  Indianapolis,  wind  west,  fresh  ;  Cleveland,  wind  south- 
west, fresh ;  Cincinnati,  Stanley,  and  Toronto,  wind  north- 
west, fresh ;  Pittsburgh,  wind  west,  brisk ;  Breckenridge, 
wind  north-east,  brisk. 

"  4.  Eainy. — At  Omaha,  calm ;  Boston,  wind  west,  light ; 
New  London,  wind  south-west,  gentle ;  Cheyenne,  wind 
north,  fresh. 

"5.  Snowy. — At  Montreal,  wind  north-east,  gentle; 
Portland,  Me.,  Kingston,  and  Quebec,  wind  north-east, 
fresh ;  Buffalo,  wind  north,  fresh ;  Dover  and  Detroit, 
wind  north-west,  fresh. 


458 


WEATHER. 


"  General  Remarks  as  to  Verifications. 

"The  above  'Probabilities'  were  generally  verified, 
except  'from  Tennessee,  northward  over  Ohio  and 
southern  portions  of  Michigan  and  Wisconsin,  winds 
shifting  to  southerly  and  easterly,  cloudy  weather,  and 
possibly  light  rain,'  and  '  for  the  North-west,  clearing  but 
partly  cloudy  weather,'  partly  verified ;  '  from  Tennessee, 
northward  over  Ohio  and  southern  portions  of  Michigan 
and  Wisconsin,  rising  temperature/  and  *  for  North-west,, 
westerly  winds,'  not  verified." 

The  foregoing  example  will  fully  illustrate  the  great 
advantage  which  the  New  England  States  possess  in  the 
ease  with  which  cyclone -depressions  can  be  traced  before 
they  reach  the  eastern  seaboard.  To  this  circumstance, 
and  to  the  energetic  management  of  the  Signal  Office,  we 
may  fairly  attribute  the  high  percentage  of  success  which 
is  achieved  in  the  United  States.  The  following  table 
gives  the  percentages  of  success  both  of  weather-forecasts 
generally,  and  of  special  storm- warnings : — 


-Year. 
1872 
1873 
1874 
1875 
1876 
1877 
1878 
1879 
1880 
1881 
1882 


These  results,  like  those  of  the  British  Meteorological 


Weather  forecasts.        Storm  warnings. 

Per  cent.                      Per  cent. 

...     76-8 

70 

77-6 

— 

...     84-4 

75 

87-4 

76 

...     88-3 

77-3 

86-2 

78-9 

...     88-4 

75'9 

90-7 

79-9 

...     90-3 

83-4 

88-7 

83-3 

...     88-2 

83-0 

FORECASTING  BY  SYNOPTIC   CHARTS.  459 

Office,  differ  little  from  year  to  year,  but  still  show  a 
slow  progressive  improvement.  A  few  details  of  the 
methods  employed  in  the  United  States  Signal  Office  will 
be  very  useful  to  show  the  practical  conditions  of  weather- 
forecasting.  They  are  compiled  from  the  publications  of 
that  office.  From  reading  in  the  morning  papers  the 
<(  Synopsis  and  Indications "  for  the  day,  no  one  not 
initiated  in  the  method  of  preparing  them  would  suspect 
the  magnitude  of  the  work  involved  in  their  elaboration. 
The  study  requisite  for  the  tri-daily  press  reports  includes 
the  drafting  of  seven  graphic  charts,  exhibiting  the  data 
furnished  by  the  simultaneous  reports  telegraphed  from 
all  the  stations,  about  seventy-five  in  number.  These 
charts  are — 

1.  A  synoptic  chart  of  pressure,  temperature,  wind's 
direction,  and  velocity,  the  state  of  the  weather,  and  the 
kind  and  amount  of  precipitation. 

2.  A  chart  of  dew-points  at  all  stations. 

3.  A  chart  of  the  various  cloud-conditions  prevailing  at 
the  time  over  the  United  States.     The  cloud-areas — each 
form  of  cloud  represented  by  a  different  symbol — are  out- 
lined, and  each  one  is  distinguished.     The  appearance  of 
the  western  sky  at  each  station  as  observed  at  sunset, 
which  affords  a  strong  indication  of  the  weather  to  be 
anticipated  for  the  next  twenty-four  hours,  is  also  marked 
in  tin's  chart. 

4.  A   chart   of  normal  barometric   pressure   and    of 
variation  of  the  actual  from  the  normal  pressures. 

5.  A  chart  of  actual  changes  of  pressure  occurring, 
showing  separately  the  fluctuations  of  the   atmosphere 
during  the  previous  eight  and  twenty-four  hours. 


460  WEATHER. 

6.  A  chart  of  normal  temperature  and  the  variations 
of  the  actual  from  the  normal  temperature. 

7.  A   chart    of    actual    changes   of    temperature   in 
previous  eight  and  twenty-four  hours. 

All  these  charts  have  to  be  made  out,  and  the  mass  of 
data  which  they  embody  to  be  sifted  and  analyzed, 
preliminary  to  the  preparation  of  every  bulletin.  Armed 
with  this  charted  material,  the  officer  preparing  the 
indications  proceeds  to  compile  the  "  Synopsis  and  Indi- 
cations," and  issue  the  necessary  storm-warnings.  The 
average  time  which  elapses  between  the  simultaneous 
reading  of  the  instruments  at  the  separate  stations,  and 
the  issue  of  the  forecasts,  is  one  hour  and  forty  minutes. 


CANADIAN  SUCCESS. 

The  following  particulars  of  the  success  obtained  by 
the  Canadian  Meteorological  Office  are  also  interesting, 
for  they  give  a  percentage  almost  identical  with  that  of 
the  neighbouring  States,  though  obtained  by  a  different 
organization. 


o 


Storm-  Warnings. 
The  percentage  of  warnings  verified  was — 

1877    ...     ...     ...     ...     ...  69-0 

3878 78-3 

1879   ' 83-0 

1880 82-8 

1881    85-0 

This  table,  like  the  others,  shows  progressive  improve- 
ment.    The  only  year  for  which  we  have  the  results  of 


FORECASTING  BY  SYNOPTIC  CHARTS.  461 

general  weather-forecasts  is  1881.  Then  the  general 
percentage  of  complete  success  was  82*3  for  the  whole 
Dominion,  while  the  proportion  of  partial  and  complete 
success  rose  to  90*2  per  cent. 

AUSTRALIAN  FORECASTS. 

We  will  conclude  this  chapter  with  an  example  of 
forecasting  in  Australia,  for  which  we  are  indebted  to 
Mr.  E.  Ellery,  of  the  Melbourne  Observatory.  This  will 
be  a  valuable  illustration  of  the  universality  of  the  general 
principles  we  have  already  laid  down.  But,  first,  let  us 
say  a  few  words  on  the  general  character  of  Australian 
weather.  The  weather  of  that  great  island  continent  has, 
like  every  other  country,  peculiarities  of  its  own,  sub- 
servient to  the  great  principles  common  to  all  the  world. 

The  same  general  distribution  of  pressure  holds  good 
there  as  elsewhere  : — a  low-pressure  zone  near  the  equator ; 
a  sub-tropical  belt  of  anticyclones  ;  an  area  of  low  pressure 
in  the  temperate  zone,  incessantly  traversed  by  an  endless 
series  of  cyclones.  Within  this  latter  area  the  same  seven 
fundamental  forms  of  isobars  are  perpetually  reproduced ; 
and  the  same  kind  of  sky  is  developed  in  the  equivalent 
part  of  each  shape  of  isobars,  and  the  same  prognostics 
hold  for  good  or  bad  weather,  as  in  the  northern  hemisphere. 
Only  the  sequence  of  the  wind  as  it  veers  during  the 
passage  of  a  cyclone  is  the  opposite  to  that  in  the  opposite 
hemisphere,  because  the  rotation  of  the  wind  round  the 
central  vortex  is  in  a  contrary  direction.  For  instance, 
we  find  the  characteristic  dirty  sky  and  muggy  heat  of 
a  cyclone  on  the  right  or  equatorial  front  in  the  northern 


462  WEATHER. 

hemisphere,  with  wind  veering  from  south-east  to  north- 
west, while  in  Australia  we  find  the  equivalent  weather 
in  the  left  (there  also  the  equatorial)  front  of  the  depres- 
sion, with  wind  beginning  at  north-east  and  going  round 
to  south-west. 

After  these  explanations,  we  can  readily  understand 
the  principles  on  which  the  following  Australian  forecasts 
were  issued  by  the  Government  Observatory  in  Melbourne. 
Let  us  look  back  at  Figs.  38  and  39,  in  which  we  give  the 
isobars  and  winds  over  all  Australia  on  November  20  and 
21,  1884. 

In  the  first  chart  (Fig.  38),  we  see  the  southern  edge 
of  the  equatorial  zone  marked  by  the  isobar  of  29*9  ins. 
over  Northern  Australia ;  the  edge  of  a  great  tropical  anti- 
cyclone lies  over  Queensland;  and  the  fragment  of  a 
temperate  cyclone  covers  the  great  Australian  Bight.  The 
wind  is  light  and  variable  at  all  the  northern  stations, 
but  rotates  round  the  cyclone  in  the  usual  manner.  Now, 
from  the  peculiarities  of  Australian  weather,  the  north-east 
or  north  winds  in  front  of  a  cyclone  of  such  moderate 
intensity  are  fine,  though  sultry,  but  occasionally  a  small 
thunderstorm  develops,  especially  near  the  trough.  The 
cyclone,  as  a  whole,  will  certainly  move  towards  the  east, 
and  the  wind  at  every  station  will  veer  according  to  the 
universal  rule. 

Hence  the  following  forecasts  were  issued  at  3  p.m. 
"South"  and  "North"  refer  to  those  portions  of  the 
colony  of  Victoria  only,  and  not  to  the  whole  of  Australia : — 

"  South.  Fine,  sultry  weather,  with  northerly  tending 
to  westerly  and  south-westerly  winds,  with  thunder  showers. 

"  North.     Ditto.  Ditto." 


FORECASTING  BY  SYNOPTIC   CHARTS.  463 

Now,  if  we  look  at  Fig.  38,  we  see  that  the  general 
anticipations  have  been  fulfilled.  The  depression  has 
moved  towards  the  east,  and  the  wind  in  Victoria  gone 
round  to  west  and  south-west.  But  a  new  anticyclone  has 
made  its  appearance  over  Western  Australia,  the  cyclone 
has  increased  in  depth,  and  thrown  out  a  V-depression 
into  the  col  between  the  two  anticyclones.  Hence  the 
intensity  has  increased,  and  the  weather  is  more  unsettled 
on  the  second  than  had  been  expected  on  the  first  day. 


INDEX. 


Abercromby,  cyclone  heat,  214 
,  deductions  from  barograms, 

393 
,  diurnal  variation  of  weather 

in  cyclones  and  anticyclones,  299 

,  monsoon  rain,  384 

,  on  prognostics,  18 

,  tropical  cyclones,  135 

Anticyclones,  26,  47,  137 

,  circulation  of,  95 

,  definition  of,  26 

,  dryness,  cause  of,  138 

,  pressure  over,  138 

,  prognostics,  47 

,  shape,  47 

Anticyclone  weather,  47 

,  antithesis  to  cyclones,  141 

wind,  47,  95. 

Aspect  of  slope,  208 
Audibility,  61 

Augustin,  rain  at  Prague,  303 
Australian  forecasts,  461 

weather,  198 

Avalanches,  effect  on  air,  239 

Salafres,  98 
Barber,  223 
Barograms,  151 

,  convex  or  concave,  394 

,  deductions  from,  393 

Barometer,  392 

,  apparent  failure  of,  399 


Barometer,  failure  of,  399 

,  fine    weather    with    low   or 

falling,  414 

,  forecasting  by  single,  393 

in  cyclone,  39 

•  in  secondary,  45 

in  squalls  and  thunderstorms, 

236 

,  jumping,  164 

,  on  board  ship,  415 

,  rain  with  rising,  401,  403 

,  rain  with  steady,  410 

Barometric    anomalies,   166,    401, 

403 

rate,  163,  395 

waves,  167 

Bebber  v.  temperature  on  cyclone 

paths,  427 
Bezold,  245 

Blanford,  Calcutta  rain,  302 
,  dependence  of  monsoon  rains, 

376 

Blizzards,  223 
"  Boen,"  248 

Break  in  the  rains  (India),  261,  386 
Breakers,  373 
British  forecasts,  441 

,  percentage  of  success,  448 

Buchan,  wind  at  sea,  305 

,  hot  and  cold  periods,  313 

Bull's-eye,  135 
Burst  of  the  monsoon,  261 
2H 


466 


INDEX. 


Calm,  183,  194 

,  centre  of  cyclone,  135 

Canadian  forecasts,  460 

Cats'  tails,  98 

Changes  of  weather,  50,  158,  294 

,  difference  from  variations  of 

weather,  158,  298 

Cirro-cumulus,  103 

(Jirro-filum,  84 

Cirro-nebula,  116 

Cirro-stratus,  100 

Cirro-velum,  101 

Cirrus,  71,  83 

,  before  barometer,  400 

,  dangerous,  9S 

,  filature,  86 

,  fine  weather,  98 

,  formation  of,  74 

,  haze,  116 

— ,  origin  of,  100 

,  over  cyclones  and  anticyclones, 

93 

,  prognostic  value,  98 

— ,  radiation  of,  88 

Cirrus-stripes,  84 

,  lie  of,  86 

, ,  relation  to  isobars,  92 

,  motion,  84,  86 

,  origin,  84 

,  relation  to  cyclones  and  anti- 
cyclones, 92 

,  etriation  of,  87,  97 

,  vanishing  points  of,  87 

Clouds,  70 

— •-,  anticyclone,  48 

— --,  cirro-cumulus,  103 

>  cirro-nebula,  1 17 

,  cirro-stratus,  100 

,  cirrus,  83 

,  cumulo-cirrus,  107 

,  cumulo-nimbus,  111 

,  cumulo-stratus,  108 

,  cumulus,  71 

• ,  cyclone,  36 

,  diurnal,  299 

,  forecasting  by,  120 

,  forma  Vioii  at  definite  levels, 

120 


Clouds,  fleecy,  103 

,  height  of,  119 

— ,  local,  282 

,  nimbus,  1 1 1 

,  nomenclature,  71 

,  perspective,  87 

,  prognostics,  70 

,  scud,  117 

,  secondary  in,  42 

,  strato-cirrus,  101 

,  strato-cumulus,  108 

,  stratus,  82 

,  striated,  97,  10  L 

,  vaults,  252 

,  woolly,  103,  114 

,  wrack,  117 

,  wreaths,  117 

Col,  147 

,  definition  of,  26 

Cold,  204 

,  great,  221 

,  in  Great  Britain,  222 

,  sources  of,  220 

Cumulo-cirrus,  107 
Cumulo-nimbus,  111 
Cumulo-stratus,  108 
Cumulus,  71,  73 

,  degraded,  80 

,  festooned,  77 

,  high,  82 

,  line,  82 

,  minor  varieties,  81 

,  relation  to  cirrus,  74 

.roll,  111 

,  turreted,  82 

Cyclical  periods  of  weather,  319 
Cyclone,  27,  125 

,  axis,  127 

,  calm  centre,  135 

,  central  eye,  135 

,  circulation  of,  93 

,  crossing  Atlantic,  421 

,  definition  of,  26 

,  double  symmetry,  32 

,  filling  up  of,  165 

,  front,  29,  38 

.  general  circulation,  127 

,  height  of,  134 


INDEX. 


467 


Cyclone,  intensity,  28 

— ,   names   of   various   portions, 

29 

paths,  419 

,  as  indicated  by  strongest 

wind,  426 

,  tendency  to  follow  cer- 
tain tracks,  420 

,  influence  of  surrounding 

temperature,  427 

,  pressure  over,  138 

,  prognostics,  27 

,  propagation,  130 

,  rain  area  of,  32 

— ,  influence  on  propagation,  132 

,  revolving,  362 

,  rear  of,  29 

— ,  sequence  of  weather  in,  39 

,  stability,  131 

,  temperature,  210 

— ,  influence  on  path,  133 

trough,  30,  178 

,   tropical   and    extra-tropical, 

135 

— ,  upper  currents,  93 

,  weather,  31 

,  winds,  31,  93 

Dappled  sky,  106 

Dependence  of  seasons,  375 

Depressions,  126 

Descriptive  records  of  weather,  180 

Dew,  51 

Diabletons,  117 

Diurnal  isotherms,  204 

Diurnal  variation,  51,  293 

,  definition  of,  51 

,  differs  for  every  shape  of  iso- 
bars, 299 

,  general  view  of  all,  310 

,    independence     of     general 

changes,  294 

,  of  ctoud,  299 

— ,  of  rain  in  cyclone,  176 
— ,  of  temperature,  210,  291 

,  of  weather,  50,  293 

,  in  anticylone,  300 

,  in  cyclone,  174,  299 


Diurnal  variation  of  weather,  differs 

in  each  shape  of  isobars,  299 
Diurnal  variation  of  rain,  301 

,  of  velocity,  170,  304 

,  of  wind,  170 

in  cyclone,  171 

,  of  direction,  171,  30 " 

,  over  sea  and  laud,  305 

Doldrums,  330 
,  weather  in1,  330 

Electricity  and  rain,  113 
Eurydice  squall.  241,  361 
Eye  of  storm,  135 

Ferrel,  200 

Festooned  cirro-cumulus,  107 

cumulus,  77 

stratus,  83 

Filature,  triangle  of,  86 
Fmley,  straight  line  gales,  189 
,  local  rains,  261 

— ,  tornadoes,  271 
Fitzroy,  103,  401 
Fohn,  219 

Force  and  velocity  of  wind,  202 
Forecasting,  390 

,  aids  to,  417 

,  checking,  449 

,  detail  possible,  431 

— ,  examples  of,  441 
from  clouds,  120 

— ,  how  far  in  advance  issued 

432 

— ,  independent  of  theory,  430 

,  nature  of  problem,  390 

,  prognostics  by,  391 

— ,  recurrent  types  and  periods, 

,  sources  of  failure,  437 

,  synoptic  charts  by,  416 

temperature,  231 

,  time  of  preparation,  433 

,  unequal  barometric  chancres 

by,  41* 

,  when  most  successful,  435 

Fracto- cumulus,  112,  117 
France,  hail  in,  289 


468 


INDEX. 


France,  thunderstorms  in.  248 
Friction,  effect  on  wind,  201 
Frost,  57 
Fuyards,  117 

Gales,  29 

,  equinoctial,  314 

,  southerly,  345 

,  straight  line,  1 89 

Germany,  forecasts  in,  449 
Globo-cumulus,  78 
Goat's  hair,  98 
Gradients,  temperature,  427 

,  wind,  183 

,  vertical  pressure,  139 

Great  cold,  221 

heat,  219 

Grouse,  373 

Hailstorms,  localization  of,  288 
Halo,  prognostic,  36,  44,  55 

,  narrowness  of  ring,  177 

Hamberg,  305 

Hann,  451 

Hazen,   tides    and  thunderstorms, 

292 
Heat,  204 

,  great,  219 

,  primary  and  secondary  effects 

of,  231 

' .  sources  of,  217 

Height  of  clouds,  119 
Hildebrandson,  cloud    names,    83, 

103,  111 

,  lie  of  stripes,  92 

,  motion  of  cirrus,  93 

,  wind  and  isobars,  193 

Hinrichs,  244 
Howard,  71,  82,  111 

,  nomenclature  of  clouds,  71 

Hurricane,  197 

India,  monsoons,  259 

,  rains,  261 

,  temperature,  219,  222 

Indian  summer,  316 
Intensity  of  weather,  29 
of  a  cyclone,  29 


Intensity  of  secondary,  46 

—  of  type,  371 

Interpretation  of  meteograms,  170 
Iowa,  squalls  in,  244 
Isobars,  7,  125 

,  configuration  of,  8 

,  origin  of,  148 

,  relations  to  wind  and  weather, 

23,  192 

,  seven  fundamental  shapes,  25 

,  what  they  are,  10 

Isobrontons,  250 
Isotherms,  diurnal,  204 
,  general,  204 

Jump  of  wind,  41,  145 

Kew,  gradients  for  wind,  186 

Khamsin,  313 

Koppeu,  on  cloud  vaults,  252 

Lammas  floods,  315 
Level  of  clouds,  120 

of  variation,  150 

Ley,  0.,  101,  122 

,  cirrus  stripes,  92 

,  cloud  names,  78,  84 

,  cumulus  tops  and  rain,  112 

,  diurnal  variation  of  wind,  305 

,  wind  and  gradients,  187 

Lightning-flash  and  rain,  114 
Line  squalls,  240 

,  cloud  vaults  in,  252 

,  with  thunderstorms,  245 

Local  variations,  280 

,  definition  of,  51 

,  of  cloud,  282 

,  of  hailstorms,  288 

,  of  rain,  261,  281 

Loomis,  93,  189,  192 
Lurid  sky,  61 

Mackerel  scales,  107 

sky,  107 

Mammato-cumulus,  78 
Mare's  tails,  28,  38,  98 
Meteograms,  151,  153 
,  interpretation  of,  170 


INDEX. 


469 


Monsoon,  313 

,  north-east,  222,  377 

, ,  temperature  of,  222 

,  south-west,  259,  381 

, .  burst  of,  261 

, ,  peculiarity  of  rain,  2GO 

, ,  temperature  of,  219 

Motion  of  clouds,  89 
Mountain  rain,  287 
Myths,  3,  181 

Nimbo-pallum,  112 

Nimbo-stratus,  112 

Nimbus,  71 

Noah's  ark,  55 

Non-instrumental  records,  180,  369 

Non-isobaric  rain,  24,  233,  259 

wind,  190 

"  Northers  "  and  "  Nortes,"  189 
Niibes  Memales,  103 
Nubiculse,  112 

Pamperos,  263 

,  clouds  in,  264 

,  dry,  263 

,  relation  to  line  squalls,  266 

• ,  sucios,  263 

,  temperature  in,  264 

Periodicities,  nature  of,  317 

Pet  day,  57 

Pocky  cloud,  78 

Poey,  78,  112 

Poudre  snow,  224 

Prague,  rainfall  of,  302 

Pressure,     distribution     over    the 

globe,  330 

Prognostics,  anticyclone,  47 
,  cannot  be  materially  advanced, 

69 

,  cyclone,  27 

,  early  explanations,  17 

,  example  of  failure,  66 

,  failure,  causes  of,  34,  51,  59, 

64 

from  damp,  35 

,  general  theory  of,  34,  64 

,  modern  developments,  64 

,  rain,  not  all  from  damp,  64 


Prognostics,  in  straight  isobars,  60 

theory  of,  64 

in  wedges,  54 

use  in  forecasting,  69,  391 

• what  they  are,  16 

will  never  be  superseded,  69 

with  dry  air,  56 

Propagation  of  cyclones,  130 

.Radiation,  effect  on  temperature, 
224 

weather,  47 

Rain,  balls,  78 

,  at  Calcutta,  302 

,  cyclonic,  261 

,  diurnal  variation,  178,  301 

,  local,  261,  284 

,  monsoon,  261 

,  mountain,  286 

,  non-isobaric,  24,  233,  259 

Rain,  preceded  by  different  prog- 
nostics, 68 
valley,  287 


,  with  calm,  45 

,  with  falling  barometer,  42 

,  with  steady    barometer,  45, 

400,  410 

with  rising  barometer,  401, 


403 

,  with  wind,  42 

Rain  prognostics,  33,  43,  56 

,  not  all  from  damp,  68 

Recurrent  types  of  weather,  312 

,  value  in  forecasting,  317 

Rainy  season,  tropics,  386 
Refraction,  58 
Ringwood,  196 
Rothesay,  rainfall,  320 

Saint  Luke's  summer,  316 

Saint  Martin's  little  summer,  316 

Saint  Medard,  315 

Saint  Swithin,  315 

Scott,  equinoctial  gales,  314 

Sea-grass,  98 

Seasonal  variations,  312 

,  definition  of,  51 

Seasons,  dependence  of,  375 


470 


INDEX. 


Seasons,  rainy,  387 

Secondary  cyclone,  definition  of,  26 

,  motion  of,  43 

Secondaries,  42 

,  thunderstorms  in,  44,  254 

,  weather  in,  43,  254 

,  wind  in,  43 

Secular  variations,  312 
Showers,  tidal,  2yl 
Simoon,  219 
Sky,  dappled,  106 

,  watery,  33 

Snow,  224,  373 
Soot  falling,  61 
Sources  of  heat,  217 
Southerly  bursters,  147 
South-west  monsoon,  250 
Southern  hemisphere,  winds,  194 
Spells  of  weather,  327 
Sprung,  246 
Squalls,  37,  233 

,  barometer  in,  236 

,  in  Iowa,  244 

,  line,  240 

,  simple,  234 

,  thunder,  235 

Stability  of  cyclones,  131 
Storms,  what,  29 

crossing  Atlantic,  421 

Straight  isobars,  59 

,  definition  of,  27 

,  prognostics  in,  60 

Strato-cirrus,  101 
Strato-cumulus,  108 
Stratus,  71,  82 

,  festooned,  83 

Striated  clouds,  87 

,  origin  of,  101 

Stripes  of  cirrus,  84 

Sun  spots  and  weather,  319 

,  value  in  forecasting,  325 

Superimposition  of    variations  on 

curves,  158 
Surge,  166 
Synoptic  charts,  7 

,  construction  of,  19 

• ,  how  developed  prognostics, 

65 


Synoptic  charts,  forecasting  by 
means  of,  416 

Temperate  zones,  weather  in,  333 

,  types  of  pressure  in,  334 

Temperature,  changes,  examples  of, 
226 

,  disturbance  of  cyclone,  213 

,  diurnal  variation  of,  210,  2U4 

,  forecasting,  230 

,  mean  diurnal  range,  296 

Theoretical  meteorology,  11 

Thermal  slope,  205,  20y 

,  aspect  of,  208 

Therm  ograms,  151 

Thunder  coming  against  wind,  256 

Thunder  heads,  81 

Thunderstorms,  233 

,  barometer  in,  236 

,  conditions  of,  255 

,  dependence  on  damp,  258 

,  independence  of  isobars,  251 

,  in  France,  249 

,  in  secondaries,  44,  254 

,  frequency  in  different  coun- 
tries, 257 

,  shape  of,  245 

,  tidal  influence,  292 

,  tracks  of,  250 

with  line  squalls,  245 

with  V-depressions,  245 

Tidal  showers,  291 

thunderstorms,  292 

wind,  291 

Tides,  irregular,  373 

Tornadoes,  263,  267 

,  cloud,  269 

,  descriptions  of,  274 

,  Fiuley  on,  271 

,  relation  to  cyclones  and  V's, 

271,  277 

,  smokiness  of,  269 

,  wind  in,  269 

Trade  winds,  nature  of,  332,  349 

,  weather  in,  331 

Trough  of  cyclone,  30,  178 

,  relation  to  velocity,  136 

of  V-depression,  144 


INDEX. 


471 


Types  of  weather,  327 

•  ,  change  of,  353,  373 

,  dependence  of,  375 

,  easterly,  363 

— ,  fluctuation  of,  353,  372 

— ,  intensity,  371 

— ,  northerly,  357 

,  persistence,  372 

,  recurrence  of,  312>  375 

,  southerly,  335 

— ,  westerly.  347 

United  States,  forecasts,  453, 
,  tornadoes,  267 

Valley  rain.  287 

Variations,  of  weather  generally, 
298 

,  cyclical,  319 

,  diurnal,  170,  293 

,  in  velocity  and  gradient,  187 

,  local,  280 

,  seasonal,  312 

,  secular  or  cyclical,  312 

Veering  of  wind,  41 

—  with  sun,  52 
Velocity  of  wind,  186 
Vertical  succession  of  air  currents, 

93,95 
Visibility,  55,  57 

with  overcast  sky,  61 

V-point  of  cloud  motion,  90 
V-shaped  depressions,  143,  240 

in  Australia,  199 

• ,  definition  of,  26 

,  two  kinds  of,  144 

Vortices,  130 

Waves,  barometric,  167 

Weather,  anticyclone,  47 

,  bad,  with  rising  barometer, 

56,  401 

— ,  Beaufort's  notation,  19 
— -  changes,  50,  158,  294 

,  cols  in,  147 

cyclone,  31 

,  dependence  of,  375 

,  diurnal  variation  of,  295 


Weather,  fine,  with  low  or  falling 
barometer,  414 

in  the  Doldrums,  330 

in  Temperate  zones,  333 

in  Trade  winds,  331 

-,  intensity,  29,  371 


,  local  variation  of,  280 

myths,  3 

,  ordinary  and  storms,  29- 

prognostics,  4 

,  radiation,  48 

,  recurrence  of,  375 

,  secondaries  in,  43 

,  spells  of,  327 

,  straight  isobar,  60 

statistics,  5 

types,  327 

,  V-depressions,  144 

variations,  51 

,  cyclical,  319 

,  diurnal,  293 

,  local,  280 

— . ,  seasonal,  312 

— ,  secular,  312 
Wedge-shaped  isobars,  53 

,  definition  of,  26 

,  prognostics,  54 

,  weather,  54 

,  wind,  54 

Weilbach,  82,  83,  103,  112 
Whipple,  186 
Whirlwinds,  263,  267 
Wind,  183 

,  anticyclone,  47 

,  barber,  223 

,  backing,  41 

,  Beaufort's  scale,  21 

,  blizzard,  223 

,  cyclone,  31 

,  diurnal  variation  of  velocitv, 

170,  304 

, ,  direction,  173,  306 

.  direction,  relation  to  gradient, 

191 

,  diurnal  variation,  173 

force,  202 

gradients,  183 

,  relation  to  velocity,  186 


472 


INDEX. 


Wind  gradients,  relation  to  direc- 
tion, 191 

,  hauling,  41 

,  inclination  to  isobars,  192 

,  keeping  down  rain,  63 

.jumping,  41,  145 

,  non-isobaric,  190 

,  relation  to  velocity  cyclone, 

200 

,  rotation  in  cyclone,  31 


Wind,  secondary,  in,  43 

,  sequence  in  cyclone,  39,  41 

,  theory  of,  200 

,  veering,  41 

velocity,    diurnal    variation, 

170 

,  relation  to  gradient,  186 

,  relation  to  force,  202 

|   Wrack,  117 

I    Wreaths  of  cloud,  117 


CNIVEKSITY  JJ 


PRINTED    BY   WILLIAM    CLOWES    AND    SONS,    LIMITED,    LONDON   AND   BECCLES. 


A  LIST  OF 

KEG  AN  PAUL,    TRENCH  &  CO.'S 
PUBLICATIONS. 


I,  Paternoster  Square, 

London. 


.A   LIST   OF 

KEGAN  PAUL,  TRENCH  &  CO.'S 
PUBLICATIONS, 


CONTENTS. 


PAGE 

GENERAL  LITERATURE .        .      2 
PARCHMENT  LIBRARY    .        .18 
PULPIT  COMMENTARY   .        .     20 
INTERNATIONAL    SCIENTIFIC 
SERIES    .        .        .        .29 


PAGE 


MILITARY  WORKS.        .        .  33 

POETRY 34 

NOVELS  AND  TALES      .        .  39 

BOOKS  FOR  THE  YOUNG       ,  41 


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Deuteronomy.  By  the  Rev.  W.  L.  ALEXANDER,  D.D.  With 
Homilies  by  Rev.  C.  CLEMANCE,  D.D.,  Rev.  J.  ORR,  D.D., 
Rev.,  R.  M.  EDGAR,  M.A.,  Rev.  D.  DAVIES,  M.A.  Fourth 
edition.  15^. 

Joshua.  By  Rev.  J.  J.  LIAS,  M.A.  With  Homilies  by  Rev. 
S.  R.  ALDRIDGE,  LL.B.,  Rev.  R.  GLOVER,  REV.  E.  DE 
PRESSENSE,  D.D.,  Rev.  J.  WAITE,  B.A.,  Rev.  W.  F.  ADENEY, 
M.A.  ;  and  an  Introduction  by  the  Rev.  A.  PLUMMER,  M.A. 
Fifth  Edition.  I2s.  6d. 

Judges  and  Ruth.  By  the  Bishop  of  BATH  and  WELLS,  and 
Rev.  T.  MORISON,  D.D.  With  Homilies  by  Rev.  A.  F.  MUIR, 
M.A.,*  Rev.  W.  F.  ADENEY,  M.A.,  Rev.  W.  M.  STATHAM,  and 
Rev.  Professor  J.  THOMSON,  M.A.  Fifth  Edition.  IDS.  6d. 

1  Samuel.  By  the  Very  Rev.  R.  P.  SMITH,  D.D.  With  Homilies 
by  Rev.  DONALD  FRASER,  D.D.,  Rev.  Prof.  CHAPMAN,  and 
Rev.  B.  DALE.  Sixth  Edition.  i$s. 

1  Kings.  By  the  Rev.  JOSEPH  HAMMOND,  LL.B.  With  Homilies 
by  the  Rev.  E.  DE  PRESSENSE,  D.D.,  Rev.  J.  WAITE,  B.A., 
Rev.  A.  ROWLAND,  LL.B.,  Rev.  J.  A.  MACDONALD,  and  Rev. 
J.  URQUHART.  Fifth  Edition.  i$s. 

1  Chronicles.     By  the  Rev.  Prof.   P.  C.  BARKER,  M.A.,  LL.B. 

With  Homilies  by  Rev.   Prof.  J.   R.   THOMSON,  M.A.,  Rev.  R. 

TUCK,  B.A.,  Rev.  W.  CLARKSON,  B.A.,  Rev.  F.  WHITFIELD, 

M.A.,  and  Rev.  RICHARD  GLOVER.     15^-. 
Ezra,  Nehemiah,  and  Esther.  By  Rev.  Canon  G.  RAWLINSON, 

M.A.  With  Homilies  by  Rev.  Prof.  J.  R.  THOMSON,  M.A.,  Rev. 

Prof.  R.  A.  REDFORD,  LL.B.,  M.A.,  Rev.  W.  S.  LEWIS,  M.A., 

Rev.  J.  A.  MACDONALD,  Rev.  A.  MACKENNAL,  B.A.,  Rev.  W. 

CLARKSON,  B.A.,  Rev.  F.  HASTINGS,  Rev.  W.   DINWJDDIE, 

LL.B.,  Rev.  Prof.  ROWLANDS,  B.A.,    Rev.  G.  WOOD,  B.A., 

Rev.  Prof.   P.  C.  BARKER,  M.A.,  LL.B.,  and  the  Rev.  J.  S. 

EXELL,  M.A.     Sixth  Edition.     I  vol.,  12s.  6d. 


22  A  List  of 

Pulpit  Commentary,  The — contimied. 

Isaiah.  By  the  Rev.  Canon  G.  RAWLINSON,  M.A.  With  Homilies 
by  Rev.  Prof.  E.  JOHNSON,  M.A.,  Rev.  W.  CLARKSON,  B.A., 
Rev.  W.  M.  STATHAM,  and  Rev.  R.  TUCK,  B.A.  Second 
Edition.  2  vols.,  i$s.  each. 

Jeremiah.  (Vol.  I.)  By  the  Rev.  Canon  T.  K.  CHEYNE, 
D.D.  With  Homilies  by  the  Rev.  W.  F.  ADENEY,  M.A.,  Rev. 
A.  F.  MUIR,  M.A.,  Rev.  S.  CONWAY,  B.A.,  Rev.  J.  WAITE, 
B.A.,  and  Rev.  D.  YOUNG,  B.A.  Third  Edition.  i$s. 

Jeremiah  (Vol.  II.)  and  Lamentations.  By  Rev.  Canon  T.  K. 
CHEYNE,  D.D.  With  Homilies  by  Rev.  Prof.  J.  R.  THOMSON, 
M.A.,  Rev.  W.  F.  ADENEY,  M.A.,  Rev.  A.  F.  MUIR,  M.A., 
Rev.  S.  CONWAY,  B.A.,  Rev.  D.  YOUNG,  B.A.  15*. 

Hosea  and  Joel.  By  the  Rev.  Prof.  J.  J.  GIVEN,  Ph.D.,  D.D. 
With  Homilies  by  the  Rev.  Prof.  J.  R.  THOMSON,  M.A.,  Rev. 
A.  ROWLAND,  B.A.,  LL.B.,  Rev.  C.  JERDAN,  M.A.,  LL.B., 
Rev.  J.  ORR,  D.D.,  and  Rev.  D.  THOMAS,  D.D.  15^. 

Pulpit  Commentary,  The.     (New  Testament  Series.) 

St.  Mark.  By  Very  Rev.  E.  BICKERSTETH,  D.D.,  Dean  of  Lich- 
field.  With  Homilies  by  Rev.  Prof.  THOMSON,  M.A.,  Rev.  Prof. 
J.  J.  GIVEN,  Ph.D.,  D.D.,  Rev.  Prof.  JOHNSON,  M.A.,  Rev.  A. 
ROWLAND,  B.A.,  LL.B.,  Rev.  A.  MUIR,  and  Rev.  R.  GREEN. 
Fifth  Edition.  2  vols.,  los.  bd.  each. 

St.  John.  By  Rev.  Prof.  H.  R.  REYNOLDS,  D.D.  With 
Homilies  by  Rev.  Prof.  T.  CROSKERY,  D.D.,  Rev.  Prof  J.  R. 
THOMSON,  M.A.,  Rev.  D.  YOUNG,  B.A.,  Rev.  B.  THOMAS, 
Rev.  G.  BROWN.  Second  Edition.  2  vols.  15^.  each. 

The  Acts  of  the  Apostles.  By  the  Bishop  of  BATH  and  WELLS. 
With  Homilies  by  Rev.  Prof.  P.  C.  BARKER,  M.A.,  LL.B.,  Rev. 
Prof.  E.  JOHNSON,  M.A.,  Rev.  Prof.  R.  A.  REDFORD,  LL.B., 
Rev.  R.  TUCK,  B.A.,  Rev.  W.  CLARKSON,  B.A.  Fourth 
Edition.  2  vols.,  los.  6d.  each. 

1  Corinthians.     By  the  Ven.  Archdeacon  FARRAR,"  D.  D.     With 

Homilies  by  Rev.  Ex-Chancellor  LIPSCOMB,  LL.D.,  Rev. 
DAVID  THOMAS,  D.D.,  Rev.  D.  FRASER,  D.D.,  Rev.  Prof. 
J.  R.  THOMSON,  M.A.,  Rev.  J.  WAITE,  B.A.,  Rev.  R.  TUCK, 
B.A.,  Rev.  E.  HURNDALL,  M.A.,  and  Rev.  H.  BREMNER,  B.D. 
Fourth  Edition.  1 5 j. 

2  Corinthians   and   Galatians.      By    the  Ven.    Archdeacon 
FARRAR,    D.D.,   and   Rev.   Prebendary  E.  HUXTABLE.      With 
Homilies  by  Rev.  Ex-Chancellor  LIPSCOMB,  LL.D.,  Rev.  DAVID 
THOMAS,  D.D.,  Rev.  DONALD  FRASER,  D.D.,  Rev.  R.  TUCK, 
B.A.,  Rev.  E.  HURNDALL,  M.A.,  Rev.  Prof.  J.  R.  THOMSON, 
M.A.,  Rev.  R.  FINLAYSON,  B.A.,  Rev.  W.  F.  ADENEY,  M.A., 
Rev.  R.  M.  EDGAR,  M.A.,  and  Rev.  T.  CROSKERY,  D.D.    Second 
Edition.     2  IT. 


Kegan  Paul,  Trench  &  Go's  Publications.         23 

Pulpit  Commentary,  The. — continued, 

Ephesians,  Philippians,  and  Golossians.  By  the  Rev.  Prof. 
W.  G.  BLAIKIE,  D.D.,  Rev.  B.  C.  CAFFIN,  M.A.,  and  Rev.  G. 
G.  FINDLAY,  B.A.  With  Homilies  by  Rev.  D.  THOMAS,  D.D., 
Rev.  R.  M.  EDGAR,  M.A.,  Rev.  R.  FINLAYSON,  B.A.,  Rev. 
W.  F.  ADENEY,  M.A.,  Rev.  Prof.  T.  CROSKERY,  D.D.,  Rev. 
E.  S.  PROUT,  M.A.,  Rev.  Canon  VERNON  HUTTON,  and 
Rev.  U.  R.  THOMAS,  D.D.  Second  Edition.  2is. 

Thessalonians,  Timothy,  Titus,  and  Philemon.  By  the 
Bishop  of  Bath  and  Wells,  Rev.  Dr.  GLOAG  and  Rev.  Dr.  EALES. 
With  Homilies  by  the  Rev.  B.  C.  CAFFIN,  M.A.,  Rev.  R. 
FINLAYSON,  B.A.,  Rev.  Prof.  T.  CROSKERY,  D.D.,  Rev.  W.  F. 
ADENEY,  M.A.,  Rev.  W.  M.  STATHAM,  and  Rev.  D.  THOMAS, 
D.D.  15.?. 

Hebrews  and  James.  By  the  Rev.  J.  BARMBY,  D.D.,  and  Rev 
Prebendary  E.  C.  S.  GIBSON,  M.A.  With  Homiletics  by  the 
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GIBSON.  And  Homilies  by  the  Rev.  W.  JONES,  Rev.  C.  NEW, 
Rev.  D.  YOUNG,  B.A.,  Rev.  J.  S.  BRIGHT,  Rev.  T.  F.  LOCKYER, 
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PUSEY,  Dr.—  Sermons  for  the  Church's  Seasons  from 
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QUEJCETT,  Rev.  W.—  My  Sayings  and  Doings.  WTith  Remi- 
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REVELL,  W.  ^.—Ethical  Forecasts.     Crown  8vo,  3^.  6d. 

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24  A  I^ist  of 

R1VINGTON,  Luke.—  Authority,  or  a  Plain  Reason  for  join- 
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28  A  List  of 

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SPECIMEN   OF  TYPE. 


4  THE  MERCHANT  OF  VENICE  ACT  I 

Salar.  My  wind,  cooling  my  broth, 

Would  blow  me  to  an  ague,  when  I  thought 
What  harm  a  wind  too  great  might  do  at  sea. 
I  should  not  see  the  sandy  hour-glass  run 
But  I  should  think  of  shallows  and  of  flats, 
And  see  my  wealthy  Andrew,  dock'd  in  sand, 
Vailing  her  high-top  lower  than  her  ribs 
To  kiss  her  burial.     Should  I  go  to  church 
And  see  the  holy  edifice  of  stone, 
And  not  bethink  me  straight  of  dangerous  rocks, 
Which  touching  but  my  gentle  vessel's  side, 
Would  scatter  all  her  spices  on  the  stream, 
Enrobe  the  roaring  waters  with  my  silks, 
And,  in  a  word,  but  even  now  worth  this, 
And  now  worth  nothing  ?    Shall  I  have  the  thought 
To  think  on  this,  and  shall  I  lack  the  thought 
That  such  a  thing  bechanc'd  would  make  me  sad  ? 
But  tell  not  me  :  I  know  Antonio 
Is  sad  to  think  upon  his  merchandise. 

Ant.  Believe  me,  no  :  I  thank  my  fortune  for  it, 
My  ventures  are  not  in  one  bottom  trusted, 
Nor  to  one  place  ;  nor  is  my  whole  estate 
Upon  the  fortune  of  this  present  year  : 
Therefore  my  merchandise  makes  me  not  sad. 

Salar.  Why,  then  you  are  in  love. 

Ant.  Fie,  fie  ! 

Salar.  Not  in  love  neither  ?    Then  let  us  say  you 

are  sad, 

Because  you  are  not  merry  ;  and  'twere  as  easy 
For  you  to  laugh,  and  leap,  and  say  you  are  merry, 
Because   yow    are   not   sad.     Now,    by   two-headed 

Janus, 

Nature  hath  fram'd  strange  fellows  in  her  time  : 
Some  that  will  evermore  peep  through  their  eyes 
And  laugh  like  parrots  at  a  bag- piper  ; 
And  other  of  such  vinegar  aspect 


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